[jorgemf]

It is very hard to know exactly what the client wants. Also it is possible the client will change its idea in the middle of the project. You basically use your experience to have an idea of how much will take you to develop the project and then add a safe margin. Where this safe margin usually is multiply per 2 or 3 of your initial estimation. You should also consider how big/complex is the company you are going to work for, the bigger the more difficult is to get what you need from them. So you multiply again per 2 or 3 if the company is big to account for the delays in getting what you need from them and other bureaucracy stuff.

The other option is to charge per hour with a range of hours you will work on the project. And if they want to make changes or add things you just charge more hours.