The dominant factor is never the technology stack — it remains how much is still undecided. Every ambiguity in the specification becomes a buffer in the estimate. A vendor that does not know what happens on the unhappy path must assume a pessimistic case. Putting two weeks into a discovery phase frequently cuts the final cost by far more than haggling over hourly rates.

Third-party integrations remain the next major multiplier. A feature that touches only your own data is predictable; the same feature connected to a payment provider and a CRM is another matter entirely. The cost lives in the counterparty: undocumented APIs, long certification processes, fields that mean something different on each side. Ask each bidder to price integrations separately, since this is where estimates break.

Quality attributes quietly rewrite the number. build an affiliate platform application used by a handful of staff has almost nothing in common with the same idea serving a hundred thousand llm integration users. Compliance work, high availability, scalability, data retention rules and localisation all add real engineering time. State them early or expect the estimate to move later.

The mix of people behind the number matters. A rate card tells you very little on its own: a senior engineer at twice the price is often less expensive in the end than a pair of junior developers who need supervision and rework. Check too what else appears on the invoice: delivery management, quality assurance, release engineering and design have to be done by someone, but they should be named rather than hidden inside a blended rate.

The number in the proposal is never the total cost. Budget for hosting, subscriptions and licences, hire dedicated react query developer logging and reactjs app development company alerting and an ongoing support budget each year. A common working assumption says that a live system requires a recurring percentage of the initial investment every year in fixes, updates and small changes. Leaving it out of the budget remains the most frequent planning error.