The single largest cost driver is rarely technology — it is how much is still undecided. Every open question in the brief is converted into a buffer in the estimate. A supplier that cannot see what is livewire in laravel happens on the unhappy path has to assume the more expensive option. Putting two weeks into a discovery phase can cut the final cost by far more than haggling over hourly rates.
Connections to other systems tend to be the next major multiplier. A feature that touches only your own data is easy to estimate; the same feature wired into an old accounting system is a different problem. The unknown sits in the third party: rate limits and sandbox access, waiting on someone else’s dedicated development team, fields that mean something different on each side. Ask each bidder to price integrations separately, because this is the usual source of overruns.
Non-functional requirements quietly rewrite the budget. An internal tool used by twenty people has almost nothing in common with the same feature set serving thousands of external customers. Compliance work, availability guarantees, scalability, data retention rules and accessibility add real engineering time. Put them in the brief or else expect the estimate to move later.
The mix of people behind the number matters. A day rate tells you very little on its own: a senior engineer at a higher rate is often less expensive in the end than two inexperienced developers who require constant review. Also ask which roles are billed: project management, QA, infrastructure work and analysis are legitimate costs, but these should be named rather than hidden inside a blended rate.
The number in the proposal is not what you will actually spend. Expect infrastructure, third-party licences, observability and an ongoing support budget each year. A common working assumption holds that a live system needs a meaningful share of its original build cost per year for updates, security patches and small improvements. Treating the launch as the finish line remains the classic mistake.