The single largest cost driver is rarely technology — it is how much is still undecided. Every open question in the specification becomes a contingency in the estimate. A team that does not know what happens on the unhappy path must assume the more expensive option. Putting two weeks into requirements work frequently cuts the overall figure by far more than any rate negotiation.
Integrations tend to be the second big 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 cost hides in the third party: poor documentation, long certification processes, fields that mean something different on each side. Ask any vendor to list every external system, as this is the usual source of overruns.
Non-functional requirements can easily double the budget. A tool used by a small internal team costs far less than the same feature set handling a hundred thousand users. Security reviews, high availability, scalability, data retention rules and localisation add real engineering time. Write them down at the start laravel or django else expect them priced as extras.
The mix of people behind the number matters a great deal. A day rate tells you very little on its own: dedicated teams vs freelancers one senior developer at twice the price is often less expensive in the end than two juniors who require supervision and rework. Check too what else appears on the invoice: coordination, testing, infrastructure work and design have to be done by someone, but they must be visible in the estimate.
The quoted figure is never the full cost of ownership. Expect infrastructure, paid APIs, monitoring and an ongoing support budget each year. A reasonable rule of thumb says that any production system requires a meaningful share of the original budget per year simply to stay current. Leaving it out of the budget has always been the most common budgeting mistake.