The single largest cost driver is rarely the technology stack — it consulting services remains unclear scope. Every open question in the requirements is converted into a contingency somewhere in the quote. A vendor that does not know the edge cases has to assume the worst. Spending a week on requirements work can cut the total much more than haggling over hourly rates.
Connections to other systems are another reliable source of cost. A form that saves data is easy to estimate; the same screen wired into an old accounting system is not. The unknown lives in the third party: rate limits and sandbox access, native app development slow approval cycles, fields that mean something different on each side. Ask the estimator to price integrations separately, as that is where the numbers slip.
Quality attributes silently change the estimate. A tool used by a handful of staff is a very different build from the same idea serving a hundred thousand users. Audit and compliance requirements, high availability, scalability, audit logging and multi-language support add measurable effort. State them early or expect the estimate to move later.
The team you are quoted changes the arithmetic. An hourly rate tells you very little on its own: one senior developer at a premium rate frequently turns out to be less expensive in the end than two juniors who require heavy code review. Also ask what else appears on the invoice: delivery management, quality assurance, release engineering and UX design are legitimate costs, but they should be visible in the estimate.
The quoted figure is not what you will actually spend. Plan flutter developers for hire hosting, paid APIs, observability and an ongoing support budget annually. A reasonable rule of thumb says that any production system consumes a meaningful share of its original build cost annually simply to stay current. Ignoring this is the most common budgeting mistake.