Scope drives the budget
The more workflows, roles, and edge cases the app needs to support, the more build time it requires. Early scope discipline is the fastest way to keep the project inside a realistic budget.
Engineering choices matter
A clean stack can reduce delivery cost over time by making the codebase easier to evolve. That is one reason many startups choose a full stack partner instead of stitching together separate vendors.
Plan for launch and iteration
A useful budget includes the first release and the next round of improvement work. That keeps the team focused on product value instead of treating launch as the end of the project.