Chapter 08 — Unit economics: what it actually costs
This is the chapter most build-in-public series skip, which is exactly why it's here. The numbers below are the real cost structure of running FoxyInvoice at its current scale, with marginal items marked as estimates where our invoices round them away. The punchline arrives early: the machine costs about a takeaway dinner a month; the real investment is your evenings. Everything after that is arithmetic.
Fixed costs: the whole business on one table
| Item | Cost (monthly) | Notes |
|---|---|---|
| VPS (entire stack, both products) | ~$8–15 | One small instance; the bottleneck is RAM, not CPU |
| Domain | ~$1 | Annual renewal amortized |
| DNS + Email routing (Cloudflare) | $0 | Free tier, honestly enough |
| Email sending (SES) | ~$0 | First 3k+ emails/mo effectively free at our volume; ~$0.10/1k after |
| Object storage (PDFs, attachments) | ~$0–2 | Kilobytes-to-megabytes per user |
| Backups off-box (S3 + email) | ~$0–1 | Compressed dumps are tiny at this scale |
| CI/CD | $0 | GitHub free tier + self-hosted runners |
| Monitoring / logs | $0 | Access logs + error console in-app; no paid APM |
| Total | ≈ $10–19/mo | ~$120–230/year, both products |
Two deliberate zeros deserve a sentence. No managed database — a $15–80/month line item at every "you should use managed Postgres" checkpoint — because our nightly, size-checked, off-box backups plus volume-backed containers cover the actual risk at this scale (Chapter 06). And no paid observability stack, because access logs plus an in-app error console answer the questions a one-person team actually asks. Managed services are answers to problems of teams you don't have; buy them when the problem arrives, not the fear.
Marginal cost: what one more free user costs
A signup costs: a few database rows (bytes), a welcome email (~$0.0001), and their invoices' PDFs in storage (cents per year). Marginal cost per free user is effectively zero. The free tier's 10-actions-per-month limit protects database write amplification and product focus — not margins. That's why "free for 10 years" is a promise we can afford to put in the terms: the cost of keeping it is measured in rounding errors.
The paid side: Stripe's take
- Payment links (invoice payments): ~2.9% + 30¢ per transaction, paid by the flow of money, not by sitting there.
- Subscriptions (Pro $9 / Business $29): the same processing rate on each charge. Net on a Pro subscription after processing: roughly $8.40/month.
Break-even arithmetic
With ~$15/month fixed:
- 2 Pro subscribers cover the entire infrastructure. Two.
- 10 Pro ≈ $84 net — the VPS, storage, a domain, and a very modest monthly "salary" for the robot.
- 100 Pro ≈ $840/mo net — real side-income territory.
- 1,000 Pro ≈ $8.4k/mo — "quit reconsidering" territory, and the scale where managed databases and paid observability do enter the budget (the cost table grows with trust, not before it).
Founding codes barely register: 6 months of Pro granted free costs nothing cash-out — it defers revenue that mostly wouldn't have existed yet, and buys testimonials, bug reports, and the honest word-of-mouth a new product can't buy any other way. Marketing spend to date: $0. The template gallery, this series, and replies where the users actually are (Chapter 09) are the marketing budget.
The cost the table can't show
Your time. The honest ledger of a solo SaaS: hundreds of evenings of build, a standing 20-minute daily cadence, and the occasional 2 a.m. incident (Chapter 13 prices those in cortisol). We don't monetize that column, but we don't hide it either — it's the actual investment everything else compounds on.
Where the revenue honesty stands today
At the time of this writing: paid subscriptions ≈ zero, users in the dozens, one power user whose feedback shaped the product more than any roadmap session. The machine is the story — a business whose cost floor is dinner-for-one monthly can afford to grow slowly and in public, which is precisely the experiment this series documents.
Recap. ≈$15/month runs two products. Free users cost nothing, which is why free-forever is a credible promise. Two Pro subscribers break even; one thousand change your life. Managed services are bought when the team-shaped problems arrive. And the real currency was evenings all along.
Next: Part IV — Chapter 09: The daily cadence — the 20-minute routine that operates the whole business.