First you need to figure out how much you're willing to donate to the charity. And how much the publicity you get from doing this is worth to you. And how much the enjoyment of the project was worth to you. Against that, figure the cost of materials plus the value of the time you spent working on it. And what the market will bear; the charity may have asked you specifically because they couldn't afford to pay much. Get agreement BEFORE you start work.Otherwise someone winds up feeling cheated, and that can ruin friendships. For my hobby-income jobs (locksmithing), I use a "flat rate manual" to compute a reasonable price as a pro would charge -- which automatically adjusts for the fact that I'm likely to take longer than someone who is doing this more often and is in better practice. I then, independently, decide how much discount I'm willing to allow, up to 100%.if the job has suffiient amusement, egoboo, or gift value. You've going to have to determine the final answer yourself; all we can do is suggest ways of approaching the question.