The dominant factor is not technology — it is almost always unclear scope. Every ambiguity in the specification becomes a buffer in the estimate. A supplier that cannot see the exceptions and edge cases has to assume the worst. Investing a few days in a discovery phase often reduces the overall figure much more than haggling over hourly rates.

Connections to other systems tend to be another reliable source of cost. A screen that writes to your own database is low risk; the same screen talking to an old accounting system is a different problem. The cost hides in the counterparty: rate limits and php developer site sandbox access, waiting on someone else's team, data that does not match your model. Ask any vendor to break integrations out as separate items, laravel inertia vs livewire because this is where estimates break.

The requirements nobody writes down quietly rewrite the budget. An internal tool used by twenty people costs far less than the same functionality handling public traffic. Audit and compliance requirements, high availability, scalability, traceability and multi-language support each add measurable effort. State them early or else expect the estimate to move later.

The mix of people behind the number changes the arithmetic. A day rate reveals very little on its own: a senior engineer at twice the price frequently turns out to be less expensive in the end than two juniors who require supervision and rework. Also ask which roles are billed: coordination, testing, release engineering and design have to be done by someone, but they should be itemised.

The quoted figure is not what you will actually spend. Budget for infrastructure, paid APIs, monitoring 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 for updates, security patches and small improvements. Ignoring this remains the classic mistake.