The single largest cost driver is never the choice of framework — it is almost always unclear scope. Every open question in the requirements becomes a buffer somewhere in the quote. A supplier that has no visibility into what happens on the unhappy path has to assume a pessimistic case. Putting two weeks into a proper discovery often reduces the final cost how much does bespoke software cost more than negotiating the rate.
Third-party integrations are the second big multiplier. A feature that touches only your own data is predictable; the same functionality connected to a payment provider and a CRM is another matter entirely. The cost lives in the third party: rate limits and sandbox access, waiting on someone else’s team, inconsistent data. Ask the estimator to price integrations separately, since that is where the numbers slip.
Quality attributes silently change the estimate. An internal tool used by twenty people costs far less than the same functionality serving public traffic. Audit and compliance requirements, availability guarantees, scalability, audit logging and accessibility all add real engineering time. State them early or hire golang developers else expect them priced as extras.
The team you are quoted changes the arithmetic. A rate card tells you very little on its own: one senior developer at a higher rate can be less expensive in the end than a pair of junior developers who require heavy code review. Also ask which roles are billed: project management, QA, infrastructure work and design are legitimate costs, but they must be named rather than hidden inside a blended rate.
The number in the proposal is rarely the total cost. Budget for infrastructure, subscriptions and licences, logging and alerting and a change budget for every year the software runs. A common working assumption is that kotlin software development company in active use consumes a recurring percentage of the initial investment per year in fixes, updates and small changes. Ignoring this has always been the most common budgeting mistake.