The next development phase includes building a user interface that is as flawless as one would imagine. That is where the importance of UI and UX design comes into being.

Since design is considered an experience, this process stage will include steps to make that experience feel and look better.

So based on how you want the application to be, designers will go about making a prototype and take up a part of your budget.
https://www.peppersquare.com/blog/how-much-does-it-cost-to-build-an...