The dominant factor is rarely the choice of framework — it remains how much is still undecided. Each unanswered question in the requirements turns into a buffer inside the number you receive. A team that does not know the exceptions and edge cases must assume the more expensive option. Investing a few days in requirements work frequently cuts the overall figure by far more than haggling over hourly rates.
Connections to other systems are the next major multiplier. A feature that touches only your own data is low risk; the same functionality talking to a legacy ERP is not. The effort lives in the third party: rate limits and sandbox access, long certification processes, fields that mean something different on each side. Ask each bidder to break integrations out as separate items, as this is where estimates break.
Non-functional requirements can easily double the budget. An application used by a handful of staff has almost nothing in common with the same idea handling a hundred thousand users. Security reviews, high availability, scalability, data retention rules and multi-language support all add measurable effort. Put them in the brief or else expect the estimate to move later.
Who actually does the work changes the arithmetic. An hourly rate says very little on its own: a senior engineer at a higher rate frequently turns out to be cheaper per delivered feature than a pair of junior hire eas developers who need constant review. Check too who else is billed: project management, testing, infrastructure work and UX design are real work, but these should be named rather than hidden inside a blended rate.
The number in the proposal is never the full cost of ownership. Plan for hosting, subscriptions and licences, observability and a change budget for every year the software development blog runs. A reasonable rule of thumb holds that software in active use consumes a meaningful share of the original budget per year for updates, security patches and small improvements. Leaving it out of the budget remains the classic mistake.