8 hrs a day 5 days a week 3 weeks 120 hours.

I would charge a minimum of $150 an hour so you are at $18k.

Most apps are just database record and forms to interact. They aren't complex.

The issue you may face is the expectation of the founder - especially if they are not technical. You will spend more time on requirements than on development.

"We need a map that shows the current location of an item plus it's status" needs to be in the next round of development. But they might not get that.