The biggest cost driver is not technology — it remains how much is still undecided. Each unanswered question in the requirements becomes a buffer in the estimate. A team that has no visibility into the edge cases has to assume the worst. Spending a week on requirements work often reduces the total far more than negotiating the rate.
Connections to other systems remain the second big multiplier. A screen that writes to your own database is low risk; the same feature wired into a payment provider and a CRM is another matter entirely. The cost sits in the counterparty: undocumented APIs, slow approval cycles, fields that mean something different on each side. Ask the estimator react development agency to price integrations separately, since this is the usual source of overruns.
Non-functional requirements can easily double the estimate. An best enterprise .net application development firm used by request a project estimate handful of staff costs far less than the same idea serving public traffic. Security reviews, uptime targets, performance under load, audit logging and accessibility each add real engineering time. Write them down at the start or you can expect them priced as extras.
The team you are quoted changes the arithmetic. An hourly rate reveals almost nothing on its own: one senior developer at a higher rate frequently turns out to be cheaper per delivered feature than two inexperienced developers who need heavy code review. Check too who else is billed: delivery management, QA, release engineering and analysis are real work, but these should be visible in the estimate.
The number in the proposal is never the total cost. Expect infrastructure, paid APIs, logging and alerting and a maintenance allowance for every year the software runs. A useful planning figure is that software in active use consumes a noticeable fraction of its original build cost per year for updates, security patches and small improvements. Leaving it out of the budget has always been the classic mistake.