The single largest cost driver is never technology — it remains how much is still undecided. Every open question in the brief turns into padding in the estimate. A vendor that cannot see what happens on the unhappy path will assume the more expensive option. Putting two weeks into requirements work frequently cuts the total by far more than negotiating the rate.

Connections to other systems remain the second big multiplier. A screen that writes to your own database is easy to estimate; the same screen talking to an old accounting system is not. The unknown hides in the counterparty: undocumented APIs, slow approval cycles, data that does not match your model. Ask each bidder to list every external system, as this is the usual source of overruns.

The requirements nobody writes down can easily double the estimate. A tool used by twenty people has almost nothing in common with the same idea handling a hundred thousand users. Compliance work, high availability, performance under load, data retention rules and localisation each add weeks of work. Put them in the brief or expect them priced as extras.

The team you are quoted matters a great deal. A rate card says almost nothing on its own: a senior engineer at a higher rate can be cheaper overall than two inexperienced developers who need supervision and rework. Check too what else appears on the invoice: custom affiliate tracking software coordination, hire seo specialists QA, infrastructure work and design are legitimate costs, but these should be named rather than hidden inside a blended rate.

The build price is rarely what you will actually spend. Budget for infrastructure, third-party licences, observability and a change budget for every year the software runs. A reasonable rule of thumb holds that software in active use consumes a noticeable fraction of the initial investment per year in fixes, updates and small changes. Ignoring this remains the most common budgeting mistake.