The dominant factor is never technology — it is almost always unclear scope. Every open question in the brief is converted into a contingency in the estimate. A vendor that has no visibility into the edge cases will assume the worst. Spending a week on a discovery phase often reduces the final cost far more than haggling over hourly rates.
Integrations are the next major python versus php multiplier. A feature that touches only your own data is predictable; the same screen talking to a payment provider and a CRM is not. The effort hides in the other system: poor documentation, waiting on someone else’s team, fields that mean something different on each side. Ask the estimator to break integrations out as separate items, since that is where the numbers slip.
Quality attributes quietly rewrite the budget. A tool used by a small internal team has almost nothing in common with the same idea serving public traffic. Security reviews, uptime targets, performance under load, audit logging and accessibility each add real engineering time. Write them down at the start or you can expect them priced as extras.
The mix of people behind the number matters. A rate card says almost nothing on its own: best react native development company a senior engineer at a higher rate is often cheaper overall than two juniors who need heavy code review. Check too what else appears on the invoice: delivery management, testing, DevOps and analysis are legitimate costs, but they must be visible in the estimate.
The build price is rarely what you will actually spend. Budget for cloud costs, paid APIs, observability and a maintenance allowance each year. A reasonable rule of thumb holds that software in active use needs a noticeable fraction of its original build cost per year for updates, security patches and hire monorepo programmer small improvements. Leaving it out of the budget is the most common budgeting mistake.