Nobody can quote an app honestly from a one-line description, and any agency that tries is quoting something other than your product. What drives the number is not the idea but the surface area: how many platforms, how many screens, how many systems it has to talk to, and how much of what it does has never been built before.

Platform comes first. One mobile platform is a build; two native builds are close to two projects. Cross-platform frameworks collapse much of that, which is why Flutter and React Native dominate first releases, but they are not free either — anything that touches device hardware or platform-specific behavior still has to be written twice.

Integrations are the line item that surprises people. A payment gateway, a CRM, an ERP, a scheduling system: each one is a contract with somebody else's software, and each one has its own auth, its own rate limits and its own failure modes. Budget for the ones you know about, then budget again for the two you will discover in month three.

The last third of the cost is the part that is invisible in a proposal — testing across real devices, production configuration, monitoring, and the first six months of fixes. Skipping it does not remove the cost. It moves it to a point where it is more expensive and more public.