I started building apps for customers and businesses.
I'm having a hard time to estimate the cost of each project.
So here's a hack to help you estimate the price you want.
1. Open a user profile in a freelancers' marketplace (oDesk and rent a coder etc.)
2. Post a request for proposals, explaining your customer's request (without giving NDA data)
3. Wait a few days and get tons of proposals
4. Calculate an average of the proposals
You now have a concrete number to play with.