[pjc50]

A lot depends on whether you're billing "time and materials" or whether there is going to be an agreed fixed price for the contract. For the second one, you have to have a pretty rigid specification in order to start - a list of all the screens and menus in an application as "wireframes", for example, with a brief description of what they do. Once you have that you can then assign some sort of average time cost to the features to produce a headline price.