The going rate for a "girlfriend experience" (making out, oral from both with a vaginal finish) with a middle aged attractive woman is between $300-$500 dollars, depending on your area. Large metropolitan areas are closer to the $500 range. The going rate for a "porn star experience" (all the above with anal thrown in) is another $100. Check the ...... sites in your area. Chances are that the man who offered you money does this regularly and is trying to get a bargain. Don't sell yourself short. A lot of men would pay extra to bust your "...... cherry".
As I re-read your post, it seems like he's offering you a "modelling fee". If it's for a website, make sure there is a contract involved and you retain some rights to your image and the money it produces.