The dominant factor is not the technology stack — it remains uncertainty. Each unanswered question in the specification becomes padding in the estimate. A vendor that cannot see what happens on the unhappy path will assume a pessimistic case. Putting two weeks into a proper discovery can cut the overall figure far more than any rate negotiation.
Third-party integrations tend to be the second big multiplier. A screen that writes to your own database is easy to estimate; the same functionality wired into a payment provider and a CRM is not. The effort sits in the counterparty: rate limits and java development company sandbox access, waiting on someone else’s team, inconsistent data. Ask any vendor to break integrations out as separate items, as that is where the numbers slip.
Quality attributes quietly rewrite the estimate. A tool used by a small internal team is a very different build from the same functionality handling thousands of external customers. Compliance work, high availability, performance under load, data retention rules and multi-language support add real engineering time. Write them down at the start or else expect them to arrive later as change requests.
Who actually does the work changes the arithmetic. A day rate reveals little on its own: an experienced engineer at twice the price is often cheaper overall than a pair of junior developers who require supervision and rework. Ask as well which roles are billed: project management, QA, infrastructure work and design have to be done by someone, laravel livewire vs react but they should be visible in the estimate.
The number in the proposal is never what you will actually spend. Budget for cloud costs, subscriptions and licences, observability and an ongoing support budget for every year the software runs. A common working assumption says that any production system consumes a noticeable fraction of its original build cost every year for updates, security patches and small improvements. Leaving it out of the budget is the most frequent planning error.