The dominant factor is never the choice of framework — it is uncertainty. Every ambiguity in the specification turns into a contingency in the estimate. A vendor that cannot see the edge cases must assume the worst. Spending a week on a discovery phase can cut the final cost by far more than negotiating the rate.
Integrations remain the next major multiplier. A feature that touches only your own data is predictable; the same screen connected to a payment provider and a CRM is a different problem. The effort sits in the third party: undocumented APIs, slow approval cycles, data that does not match your model. Ask each bidder to break integrations out as separate items, as this is the usual source of overruns.
Quality attributes quietly rewrite the estimate. A tool used by a small internal team costs far less than the same feature set handling a hundred thousand users. Security reviews, availability guarantees, ai in custom software development performance under load, audit logging and localisation all add real engineering time. Write them down at the start or else expect them to arrive later as change requests.
The team you are quoted matters. A rate card says little on its own: laravel inertia vs livewire one senior developer at twice the price is often less expensive in the end than two inexperienced developers who need supervision and rework. Also ask what else appears on the invoice: project management, quality assurance, DevOps and design are real work, but these should be itemised.
The quoted figure is rarely the total cost. Budget for cloud costs, third-party licences, monitoring and an ongoing support budget annually. A common working assumption says that a live system consumes a recurring percentage of its original build cost annually simply to stay current. Treating the launch as the finish line remains the classic mistake.