The single largest cost driver is not the technology stack — it remains unclear scope. Every ambiguity in the specification is converted into a contingency inside the number you receive. A vendor that cannot see the exceptions and edge cases has to assume a pessimistic case. Putting two weeks into requirements work can cut the total by far more than haggling over hourly rates.
Integrations are the next major multiplier. A form that saves data is predictable; the same functionality connected to an old accounting system is another matter entirely. The unknown lives in the counterparty: rate limits and why mvps fail sandbox access, long certification processes, data that does not match your model. Ask any vendor to break integrations out as separate items, because this is where estimates break.
Non-functional requirements silently change the budget. An internal tool used by a handful of staff has almost nothing in common with the same idea serving public traffic. Security reviews, uptime targets, load handling, audit logging and multi-language support add measurable effort. Write them down at the start or else expect them priced as extras.
The team you are quoted matters. A day rate tells you very little on its own: an experienced engineer at a higher rate can be cheaper per delivered feature than two juniors who require heavy code review. Check too who else is billed: coordination, testing, laravel vs django comparison infrastructure work and design are legitimate costs, but these should be itemised.
The build price is rarely what you will actually spend. Budget for cloud costs, paid APIs, observability and a maintenance allowance annually. A reasonable rule of thumb is that any production system consumes a recurring percentage of the initial investment per year simply to stay current. Ignoring this has always been the most common budgeting mistake.