The dominant factor is never the choice of framework — it is almost always uncertainty. Every open question in the brief turns into a contingency inside the number you receive. A supplier that has no visibility into the exceptions and edge cases must assume a pessimistic case. Spending a week on a discovery phase often reduces the final cost far more than negotiating the rate.
Integrations are the next major multiplier. A feature that touches only your own data is predictable; the same functionality wired into an old accounting system is a different problem. The cost hides in the third party: undocumented APIs, long certification processes, fields that mean something different on each side. Ask each bidder to list every external system, since this is the usual source of overruns.
The requirements nobody writes down silently change the budget. An internal tool used by twenty people has almost nothing in common with the same feature set handling a hundred thousand users. Security reviews, availability guarantees, scalability, audit logging and localisation all add measurable effort. State them early or you can expect the estimate to move later.
The team you are quoted matters a great deal. A rate card says very little on its own: an experienced engineer at a higher rate can be cheaper per delivered feature than a pair of junior hire professional laravel developers who need constant review. Ask as well who else is billed: project management, flutter development services testing, infrastructure work and analysis are legitimate costs, but they must be itemised.
The number in the proposal is not what you will actually spend. Plan for cloud costs, third-party licences, monitoring and a maintenance allowance for every year the ecommerce software development company runs. A reasonable rule of thumb holds that a live system requires a recurring percentage of the original budget per year in fixes, updates and small changes. Treating the launch as the finish line remains the most common budgeting mistake.