The cost of building your app supposed to be zero because its your idea and you are going to build it?
Marketing and other development funds can be extra and it depends on which platform you are choosing to promote your final product.
You're thinking about an ICO/IEO/Defi?
It's not zero, it's very costly to make one idea into reality if it's about blockchain related project. You have to hire developers if you can't code and it's hard to do it on your own, alone.
How about budgeting it from $20,000 - $100,000 just for the development?