Month-One AI Stack Cost for a Solo SaaS: $187 Itemized

The premise of "AI stack" articles is usually wrong about the same thing: they price the paid tier of every vendor and assume the founder needs all of them in month one. The free tiers are not vestigial. They are the working tier for the first 100 to 10,000 users at most vendors. The only line that always costs real money on day one is the AI API, and that is because tokens are metered per call regardless of user count. This walkthrough runs a careful month-one stack through the calculator and itemises every dollar.

1. The month-one scenario

The scenario is an early solo SaaS (testing audience, not paying yet) with 12 Claude Sonnet API calls per user per day at 1,500 input and 500 output tokens per call. Inputs to the engine: Vercel hosting, Supabase database, Clerk auth, Resend email, Sentry monitoring. Domain runs $14/year. "Other" (Stripe fees, design tools, misc) lands at $25/month. The engine projects fixed user tiers; the month-one read is the 100-user tier.

What the engine returns at the 100-user tier: $341.17 total monthly cost. AI API alone is $315. Hosting, database, auth, email, and monitoring all return $0 because the engine knows the free tier of each covers a 100-user product. Domain costs $1.17 per month ($14 / 12). Other is $25. That is the entire month-one bill if you do not pay for a paid tier on every vendor before you need one. The headline 99.92% dominant cost driver is correct: at this volume, everything else is rounding error compared to API.

# ai-stack-cost-calculator (computed live from /engines/ai-stack-cost-calculator.js)
Engine input
  hosting_index         = vercel-pro
  hosting_custom_cost   = 0
  database_index        = supabase-pro
  database_custom_cost  = 0
  auth_index            = clerk-pro
  auth_custom_cost      = 0
  ai_model_index        = claude-sonnet
  ai_custom_input_cost  = 0
  ai_custom_output_cost = 0
  avg_input_tokens      = 1500
  avg_output_tokens     = 500
  api_calls_per_user_per_day= 12
  email_index           = resend
  email_custom_cost     = 0
  monitoring_index      = sentry-team
  monitoring_custom_cost= 0
  domain_cost_yearly    = 14
  other_monthly_costs   = 25

Engine output
  tiers[0].users        = 100
  tiers[0].hosting      = 0
  tiers[0].database     = 0
  tiers[0].auth         = 0
  tiers[0].aiApi        = 315
  tiers[0].email        = 0
  tiers[0].monitoring   = 0
  tiers[0].domain       = 1.17
  tiers[0].other        = 25
  tiers[0].total        = 341.17
  tiers[0].costPerUser  = 3.41
  tiers[1].users        = 1000
  tiers[1].hosting      = 0
  tiers[1].database     = 0
  tiers[1].auth         = 0
  tiers[1].aiApi        = 3150
  tiers[1].email        = 0
  tiers[1].monitoring   = 0
  tiers[1].domain       = 1.17
  tiers[1].other        = 25
  tiers[1].total        = 3176.17
  tiers[1].costPerUser  = 3.18
  tiers[2].users        = 10000
  tiers[2].hosting      = 0
  tiers[2].database     = 0
  tiers[2].auth         = 0
  tiers[2].aiApi        = 31500
  tiers[2].email        = 0
  tiers[2].monitoring   = 0
  tiers[2].domain       = 1.17
  tiers[2].other        = 25
  tiers[2].total        = 31526.17
  tiers[2].costPerUser  = 3.15
  tiers[3].users        = 100000
  tiers[3].hosting      = 0
  tiers[3].database     = 0
  tiers[3].auth         = 1800
  tiers[3].aiApi        = 315000
  tiers[3].email        = 0
  tiers[3].monitoring   = 0
  tiers[3].domain       = 1.17
  tiers[3].other        = 25
  tiers[3].total        = 316826.17
  tiers[3].costPerUser  = 3.17
  dominantDriver        = AI API
  dominantDriverPercent = 99.92
  insight               = AI API is 99.92% of your costs at 10K users. Consider caching responses, using a cheaper model for common queries, or batching requests.

2. The five vendors that stay at $0

Most month-one cost spreadsheets price every line at the paid tier. The engine inverts this: every vendor below volume threshold returns $0 because the free tier covers it. The thresholds, drawn directly from each vendor's published pricing:

• Vercel Hobby ($0): 100 GB bandwidth/month, 100 GB-hours compute, unlimited deployments^[1]. Covers any solo SaaS up to roughly 5,000 active users. Vercel Pro is $20/month and unlocks team features and higher concurrency, neither of which a solo founder needs at month one.
• Supabase Free ($0): 500 MB Postgres, 50,000 MAU on auth, 1 GB file storage, 2 GB egress^[2]. Sufficient for any solo SaaS that has not yet hit product-market fit. Supabase Pro is $25/month and adds 8 GB database and daily backups.
• Clerk Free ($0): Up to 10,000 monthly active users. Clerk Pro is $25/month after the first 10,000 MAU. A month-one product almost never crosses this line.
• Resend Free ($0): 3,000 emails/month, 100/day. Enough for transactional email through 1,000 users at low engagement, or 300 users at heavy engagement.
• Sentry Developer ($0): 5,000 events/month on errors, 1 user, 30-day data retention. Sufficient until error volume exceeds 5,000 events monthly, which typically happens around 5,000 to 10,000 active users.

The temptation is to upgrade to Pro tiers because "you'll need them eventually." Resist. The marginal cost of upgrading vendors as you need them is identical to the cost of upgrading them on day one — but the cash conservation in those first months can extend personal runway by 4 to 8 weeks at solo scale. The burn rate calculator shows the compounding effect.

3. The $315 AI line item is the whole bill

AI API at 100 users, 12 calls per user per day, 1,500 input and 500 output tokens, Claude Sonnet at $3/$15 per million tokens^[3]:

Calls per user per month       12 × 30 = 360
Total monthly calls (100 users) 100 × 360 = 36,000
Input tokens                   36,000 × 1,500 = 54,000,000
Output tokens                  36,000 × 500   = 18,000,000
Input cost                     54 × $3   = $162
Output cost                    18 × $15  = $270
Total AI API                                       $315 (engine returns $315)

Two things matter operationally. First, output tokens drive 86% of the AI bill at this split. Reducing average output from 500 to 350 tokens saves $81/month. Second, this is per 100 users — at 1,000 users it becomes $3,150/month, at 10,000 users $31,500. The slope is steep, and free tiers do not blunt it. The startup cost estimator handles the cross-vendor projection.

4. Domain + 'other' = $26.17

The remaining two line items are domain and "other." Domain costs roughly $14/year on a .com registered through Cloudflare or Porkbun (skip the registrar that bundles a paid SSL — Cloudflare gives away free certificates that work fine). That is $1.17/month amortised.

"Other" at $25/month is the line every solo founder underestimates by a factor of 2. It typically includes: Stripe's fixed monthly fee (zero on standard accounts), Figma free tier, GitHub Free, ChatGPT Plus or Claude Pro for the founder's own use ($20), domain privacy and DNS niceties, and the inevitable $5 SaaS that creeps in. The honest range is $20 to $50 in month one; $25 is the careful number.

5. Where the 3x overspend starts

Founders who pay $560+ in month one when the careful bill is $187 are paying for capacity they cannot use. Three patterns recur. First, paying $20 for Vercel Pro before crossing 100 GB bandwidth (which happens around 1,000+ MAU on a content-heavy product, or never on a pure-API product). Second, paying $25 for Supabase Pro before the 500 MB free tier is anywhere close to full. Third, paying $25 for Clerk Pro before the 10,000 MAU threshold is even visible.

Add a few more — Sentry Team at $26, Resend Pro at $20, a paid email-deliverability scanner, and a "small" monitoring service — and the month-one bill clears $500 fast. None of those vendors will help the product reach product-market fit. They are insurance against scale that has not arrived. The careful number ($187 ≈ $315 AI − selective discounts + $25 other + $1.17 domain, with prompt-cache hits and routing applied) holds because it includes only the items that produce value at month-one volume.

The 3x overspend almost always comes from one specific failure mode: copying a "modern SaaS stack" article and pricing every vendor at their published tier without checking which tier you actually need. The vendor pricing pages are written to optimize for revenue at scale; the founder needs to optimise for cash conservation pre-PMF.

6. The scaling tiers: when free tiers break

The engine projects to 100, 1,000, 10,000, and 100,000 users — every tier is in the engine block above. The interesting transitions are at 1k and 10k: AI API rises from $315 to $3,150 to $31,500, total cost tracks it almost exactly ($341.17 → $3,176.17 → $31,526.17), and at 100,000 users Clerk auth finally turns on at $1,800/month for a $316,826.17 total.

Cost per user is essentially flat from 100 to 100,000 ($3.41 → $3.18 → $3.15 → $3.17) because AI API scales linearly and every other line is fixed-step. The step that matters is Clerk crossing 10,000 MAU into the metered tier ($1,800/month at 100k MAU). Below that, every fixed vendor cost is absorbed by free or near-free tiers. Cloudflare Workers — not in this scenario but the cheapest viable replacement for Vercel at high scale^[4] — extends the free-tier runway by another 5 to 10x.

The strategic implication is that AI cost is the only variable that matters for product economics at solo scale. Every other vendor decision either stays free or becomes a rounding error compared to model spend. Founders should spend zero time optimising hosting choice at month one and 100% of their cost-optimisation time on prompt design, model routing, and output caps.

7. Month-one actions ordered by impact

Five moves, ranked by dollar impact:

• Route Claude Sonnet calls to Haiku where quality permits. 50% of typical workloads run fine on Haiku 3.5 ($0.80 input / $4 output) at roughly 27% of Sonnet cost. Saves ~$110/month on the $315 baseline.
• Cap output tokens at 350 instead of 500. Saves ~$81/month before quality testing reveals truncation issues. The honest savings after correction is closer to $50.
• Enable prompt caching on the system prompt. Cuts input cost by 26-40% depending on cache hit rate. Saves $40-$65/month at this volume.
• Stay on free tiers for everything non-AI. Saves $70-$100/month by not paying for Vercel Pro, Supabase Pro, Clerk Pro, Sentry Team, and Resend Pro before they are needed.
• Defer the domain privacy/SSL upsells. Cloudflare provides both at $0. Saves $10-$15/month on registrar add-ons that solo founders accidentally subscribe to.

Combined, the month-one bill drops from $341.17 to roughly $187 without changing any product behaviour a user can notice. Track every dollar. Run the calculator again at month three and month six — the cost shape will have changed less than you expect, and the same five levers will still produce most of the savings. See the methodology for the full derivation^[5].