The single largest cost driver is rarely the choice of framework — it remains how much is still undecided. Every ambiguity in the specification turns into padding in the estimate. A supplier that has no visibility into the exceptions and edge cases has to assume the more expensive option. Putting two weeks into a discovery phase often reduces the total by far more than haggling over hourly rates.
Integrations tend to be the second big multiplier. A screen that writes to your own database is predictable; the same screen wired into an old accounting system is a different problem. The effort hides in the other system: rate limits and erp development company sandbox access, long certification processes, fields that mean something different on each side. Ask the estimator to break integrations out as separate items, because that is where the numbers slip.
The requirements nobody writes down can easily double the number. A tool used by twenty people has almost nothing in common with the same idea handling a hundred thousand users. Audit and compliance requirements, availability guarantees, net cloud development load handling, audit logging and localisation all add weeks of work. Put them in the brief or you can expect the estimate to move later.
The mix of people behind the number matters. An hourly rate reveals little on its own: one senior developer at twice the price can be cheaper per delivered feature than a pair of junior developers who require heavy code review. Ask as well what else appears on the invoice: project management, quality assurance, DevOps and design are real work, but they should be itemised.
The quoted figure is not the total cost. Expect cloud costs, paid APIs, observability and a maintenance allowance for every year the software development for fintech runs. A common working assumption holds that a live system needs a meaningful share of the initial investment every year simply to stay current. Leaving it out of the budget is the most common budgeting mistake.