This is a good place to start; I've done this for several freelance projects, start with a "Feasibility Study" and charge a fixed price for it. This gives you

This is a good place to start; I've done this for several freelance projects, start with a "Feasibility Study" and charge a fixed price for it. This gives you time to investigate the effort involved in the work. The deliverable is a short document outlining what the scope of your solution is and ideally how much it will cost (for small projects clients generally prefer a fixed price). Pick a number that you think you would be happy doing the work for and you think the client will be willing to pay- this could be based on estimated time x rate or simply a number you are comfortable with. If this is agreed then you can move forward with doing the actual work.