The biggest cost driver is not the choice of framework — it is almost always uncertainty. Every open question in the brief turns into a buffer in the estimate. A vendor that has no visibility into the exceptions and edge cases has to assume a pessimistic case. Spending a week on a proper discovery frequently cuts the total much more than negotiating the rate.
Third-party integrations are the second big multiplier. A form that saves data is low risk; the same functionality connected to a payment provider and a CRM is not. The unknown hides in the third party: poor documentation, kotlin development agency long certification processes, data that does not match your model. Ask the estimator to list every external system, because that is where the numbers slip.
Non-functional requirements quietly rewrite the budget. An application used by twenty people is a very different build from the same idea serving a hundred thousand users. Compliance work, uptime targets, backend and frontend technologies we use performance under load, audit logging and accessibility add weeks of work. State them early or you can expect them to arrive later as change requests.
The team you are quoted matters. A rate card tells you little on its own: one senior developer at twice the price is often cheaper per delivered feature than two juniors who require heavy code review. Also ask who else is billed: how much does it cost to outsource software development project management, testing, release engineering and analysis are legitimate costs, but these should be visible in the estimate.
The quoted figure is never the total cost. Expect cloud costs, paid APIs, logging and alerting and a change budget annually. A common mvp mistakes working assumption says that a live system consumes a meaningful share of the original budget per year in fixes, updates and small changes. Leaving it out of the budget remains the classic mistake.