The dominant factor is rarely technology — it is almost always unclear scope. Every open question in the brief is converted into a contingency in the estimate. A supplier that has no visibility into what happens on the unhappy path must assume the more expensive option. Spending a week on a proper discovery can cut the total by far more than negotiating the rate.
Third-party integrations are the second big multiplier. A feature that touches only your own data is easy to estimate; the same feature talking to an old accounting system is not. The effort lives in the other system: poor documentation, slow approval cycles, fields that mean something different on each side. Ask any vendor to list every external system, since this is the usual source of overruns.
Non-functional requirements silently change the estimate. A tool used by a handful of staff has almost nothing in common with the same idea serving public traffic. Compliance work, high availability, load handling, traceability and accessibility each add measurable effort. State them early or expect them to arrive later as change requests.
Who actually does the work changes the arithmetic. A rate card tells you almost nothing on its own: an experienced engineer at a premium rate is often less expensive in the end than two inexperienced developers who need heavy code review. Ask as well which roles are billed: delivery management, QA, aws software development company release engineering and design are real work, but these should be visible in the estimate.
The number in the proposal is never what you will actually spend. Budget for hosting, subscriptions and licences, logging and alerting and a maintenance allowance for every year the custom government software development runs. A common working assumption is that any production system consumes a recurring percentage of the initial investment every year for updates, security patches and small improvements. Treating the launch as the finish line remains the most common budgeting mistake.
