SaaS billing platform decisions are tactical until they hit scale; then they’re strategic. The counterintuitive claim: for many B2B products the buyer-vendor default (Stripe + a billing SaaS) is higher-risk on margins and differentiation than a targeted in-house build once you cross predictable volume and complexity thresholds.
Direct answer: If you process under $50k/month gross revenue and you need standard recurring billing, buy Stripe plus a billing vendor (Recurly, Chargify, Paddle) — expected 3-year TCO falls under $150k and time-to-revenue is weeks. If you run over $250k/month, require nested usage-based metering, or need custom invoice workflows, build: a 3-engineer 9–12 month project (~$540k–$720k fully loaded) will typically pay back in 12–24 months on reduced vendor fees and new revenue capture.
The stakes are dollars and time. Stripe’s baseline card fee is 2.9% + $0.30 per transaction in the US; Paddle and other full-stack vendors charge effective take rates of 3%–7% for payments plus platform services. A 1% difference in take rate on $3M ARR is $30k/yr. A 3-engineer team in the US runs roughly $540k–$660k/year loaded. Those two numbers determine whether build ever makes financial sense.
Beyond pure fees there are operational costs: PCI scope, chargeback handling, tax and VAT automation, invoice templates for enterprise contracts, dunning sophistication, and revenue recognition. SaaS vendors cover most of this but at a price and with limited policy control; in-house solutions give control at the cost of engineering time and ongoing maintenance.
SaaS billing platform: vendor vs. in-house tradeoffs
Cost math is simple to model and decisive. Example A: buy. Stripe + Recurly on a mid-market plan often runs $2k–$8k/month plus Stripe fees. On $100k/month gross revenue that’s roughly $24k–$96k/yr vendor fees + Stripe processing fees ~ $34k/yr (2.9% baseline) — total ~ $58k–$130k/yr. Example B: build. A focused 3-engineer team for 9 months costs $405k–$540k (engineer loaded $150k–$200k/yr). Add $50k/yr SRE and compliance run rate. Year 1 TCO favors buy; years 2–3 the build can cross even if the vendor fees are moderate.
Use-case complexity matters more than raw ARR. If you offer nested metering (per-customer subaccounts with independent quotas), multi-entity revenue recognition, or real-time credit allocation for marketplace payouts, the engineering scope multiplies. Each advanced feature typically costs 3–8 developer-weeks to implement correctly and another 2–4 weeks per customer for integration and contracts. Vendors rarely offer that level of custom workflow without an enterprise price tag: Zuora and Stripe Billing enterprise deals commonly start at $10k–$30k/month.
Operational risk is underpriced by buyers. Disputes and chargebacks cost more than fees; a 0.5% dispute rate on $5M ARR at $1.5k average dispute handling cost is $37.5k/yr in operational overhead. Vendors automate part of this but keep you in a standardized flow. Building gives you policy control — e.g., custom dispute triage that reduces loss rate by 0.2 percentage points — but you'll need a payments ops hire ($120k–$180k/yr) and engineering support.
Treat billing as a product decision: buy to accelerate market fit; build when fees, product differentiation, or regulatory control exceed an explicit dollar threshold.
How to calculate the three-year TCO and build decision
Step one: quantify vendor effective take-rate. Combine payment processing (Stripe 2.9% + $0.30), billing vendor subscription ($2k–$15k/month), and platform transaction surcharges (Paddle/Chargify can add 1%–3%). For a company at $200k ARR ($16.7k/mo) a 1% delta in take-rate is $2k/yr; at $3M ARR it’s $30k/yr. Put that number into a 3-year net present value calculation at your cost of capital (use 8%–12% for startups).
Step two: add build and run costs. A minimal in-house billing stack typically requires: 2 backend engineers, 1 SRE/infra, and one payments/ops engineer in year 1 — loaded cost roughly $850k–$1.1M. Add cloud costs: Postgres, ledger storage, background workers, and egress — estimate $4k–$12k/month at mid-volume. Ongoing maintenance is ~20% of initial build cost per year, which you must include in the 3-year TCO.
Step three: include switching and opportunity costs. Migrating customers off a vendor rarely runs under 2–4 months of engineering and product time and often requires customer success hours. Put $75k–$250k as a conservative switching cost. Also include the opportunity cost of the engineering team: that same 3-person team could deliver two core product features per quarter that lift retention by 1–3 percentage points — quantify that in ARR impact.
What this means for CTOs and technical founders
If you’re pre-product-market fit and revenue is below $50k/month, buy. Time-to-revenue matters more than marginal fee savings. A buy-stack (Stripe + Paddle or Stripe + Recurly) gets you contract management, PCI scope reduction, tax handling, and revenue recognition in weeks, not months. Expect implementation work of 2–6 engineering sprints and monthly vendor spend below $5k in year one.
If you’re post-PMF and revenue trends above $250k/month or you’re pursuing differentiated pricing (usage-based tiers, negative-balances, partner payouts), build. At $250k/mo gross revenue a 1.5% reduction in take-rate equals $45k/yr. A focused build that cuts vendor exposure by 2% pays back within 18–24 months when you factor in new revenue enabled by custom metering and enterprise invoice workflows.
Hybrid is the pragmatic path for many companies: keep Stripe (payments) and an off-the-shelf billing vendor for customer-facing self-service, but implement a lightweight in-house metering service for your most valuable customers. This reduces vendor lock-in and gives you control where the revenue lift happens without investing six figures upfront across the entire stack.
Checklist to decide (3–5 items)
1. Calculate your vendor effective take-rate and translate a 1% change into dollars on your current ARR and 18-month ARR projection.
2. List the non-standard billing features you require (nested tenants, marketplace payouts, custom proration, revenue recognition) and estimate engineering work in developer-weeks.
3. Add fixed costs: PCI compliance scope reduction benefits, payments ops headcount, and switching cost; treat switching as a 2–4 month engineering project with $75k–$250k budget.
4. Run a 3-year TCO with a conservative 20% maintenance drag on build cost and compare against vendor spend + Stripe fees; prefer buy unless build pays back within 24 months or unlocks >$200k/yr of new revenue.
5. If you choose hybrid, define clear ownership boundaries: vendor for self-serve flows, in-house for enterprise metering and reconciliation, and a migration playbook for eventual consolidation.
The decision is rarely binary. A startup that sold through self-serve channels and hit $12k MRR in months should have bought. The same company that wins several enterprise deals with complex SOWs will naturally shift to build once the numbers line up. The engineering trade-offs are straightforward — control and margin versus speed and headcount opportunity cost.
If you decide to build, adopt these engineering constraints: design an immutable ledger for revenue events, use a streaming architecture (Kafka or managed alternatives) for invoice generation under load, and isolate billing state into a single writable service with read-only replicas for analytics. That architecture minimizes reconciliation headaches and keeps latency for customer-facing invoice queries under 200ms.
If you decide to buy, negotiate the SLI/SLOs and exportability clauses. Ensure the vendor gives you raw event webhooks, full transaction exports in Parquet/CSV, and a contractual 30–90 day data export SLA. Those clauses cut future switching costs by a large multiple.
The choice to build billing is not a romantic engineering bet. It’s a financial lever. When vendor fees, product differentiation, or compliance control sum to a predictable dollar that exceeds your fully-loaded engineering cost over a 12–24 month horizon, build. Otherwise, buy and focus engineers on product retention and growth.
