The two dominant development engagement models — fixed-price sprint and time-and-materials marathon — produce completely different risk profiles, incentive structures, and outcomes. For a first AI MVP, this choice is often the difference between shipping and running out of runway.
The Comparison
Fixed-Price Sprint
A time-boxed engagement (typically 2–4 weeks for an MVP) with a defined scope and a fixed total cost agreed upfront. The vendor absorbs estimation risk. SpeedMVPs exclusively uses this model.
- Know exactly what you'll pay, receive, and when before signing
- Vendor absorbs estimation risk — if they underscoped, their margin suffers, not yours
- Structural incentive to finish fast — the vendor makes no more money by going slower
- Budget certainty for pre-revenue founders where every dollar counts
- Forces good upfront scoping discipline from both sides
- ×Scope must be reasonably well-defined before kickoff
- ×Mid-build changes require a formal change order process
- ×Not suited for genuinely open-ended R&D with unknowable scope
- ×Fewer vendors offer true fixed-price than claim to
T&M Marathon
Time-and-materials billing charges for every hour worked, regardless of output. Vendors have no structural incentive to finish quickly — every meeting, scope debate, and architecture discussion is billable time.
- Flexibility to change direction mid-build without formal change orders
- Appropriate for large enterprise projects with genuinely unknowable scope
- Works for long-term retainer relationships with deep product knowledge
- Better fit for ongoing maintenance after initial build is complete
- ×Client absorbs 100% of estimation risk — if the build takes twice as long, you pay twice
- ×Vendor is financially rewarded for taking longer (misaligned incentives)
- ×Scope creep is structurally encouraged by the billing model
- ×Budget uncertainty is existential for pre-funded startups
- ×Meetings, architecture debates, and 'nice to haves' are all billable time
Risk and cost comparison
| Factor | MVP Approach | Alternative |
|---|---|---|
| Cost predictability | Fixed-price: 100% — agreed before signing | T&M: variable — unlimited upside risk |
| Estimation risk | Fixed-price: vendor bears the risk | T&M: client bears 100% of risk |
| Incentive alignment | Fixed-price: vendor incentivized to ship fast | T&M: vendor financially rewarded for going slower |
| Change handling | Fixed-price: transparent change order process | T&M: any change is just billed |
| Discovery cost | Fixed-price: included in engagement | T&M: $10k–$30k before build starts |
| Best for | Fixed-price: first MVP, pre-revenue founders | T&M: enterprise with internal team, ongoing maintenance |
Key Takeaways
- For your first AI MVP: fixed-price sprint, every time. T&M is appropriate only for enterprises with internal teams and deep vendor relationships.
- The incentive problem with T&M is structural: a 3-week build at $150/hr costs $18k. At 8 weeks it costs $48k — the vendor earns $30k more for taking longer.
- Fixed-price puts estimation risk on the vendor; T&M puts it on you. For a pre-revenue startup, you cannot afford to absorb that risk.
- Good fixed-price studios handle scope changes through a defined change order process — fast, transparent, and reasonably priced.
- For ongoing iteration after an MVP ships, structured sprint retainers (fixed deliverables per sprint) beat open-ended T&M retainers.
Who should use which model
Pre-seed founder
Fixed-price always. T&M is a runway risk you cannot absorb without an established track record with the vendor.
Seed-stage founder
Fixed-price for MVP scope; structured sprint retainer (fixed per-sprint) for post-launch iteration.
Enterprise innovation team
T&M acceptable only if you have embedded engineers managing the vendor. Fixed-price for well-defined use-case MVPs.
CFO / finance lead
Fixed-price converts an unbounded liability into a known cost — dramatically simpler to approve and track.
First-time founder evaluating agencies
Fixed-price first engagement always. Use T&M only after you've shipped together and built trust.