How much does it cost to build an app?
It depends... Some apps can be created quickly. I created this one in just 26 hours. However, some apps cost millions. It can be anything in between.
As a startup, you may want to keep it cheap and efficient. To keep it as cheap as possible, I think the following principles should be thought through.
Keep it simple, focus on your USP. Don't recreate other things, imitation is procastrination https://medium.com/@smuzani/a-root-cause-of-startup-death-a7850b2c2e35. I've made this mistake many times.
Use React Native (Web), instead of Native Android, Native iOS, and some Web Framework.
Keep your development environment simple. Preferably, use just one language.
Premature scaling is the root of all evil.
If you want to, I can give you an estimation... Send me a message!