For a custom modelling job you need to work out your actual costs, then add a profit margin. You need to cover, your time, computer wear and tear, electricity, building costs etc. For a hobbiest making a little extra cash I would consider things like, did I enjoy doing it, would I have done something similar anyway, is it for a friend, is he using it commercially etc. Charging $10 per hour would be very low end, $150 per hour more top end. If you can make a few small changes and sell it in the marketplace charge him for your time and the marketplace is mostly profit.