First time producing art for monetary compensation. It's a good 400px x 200px. I spent a little over 3 hours on it. My client loved it, asking "how much money you want for your excellent work." I have no idea how much to charge. I don't want to rip off my client but at the same time I don't want to get ripped off. I'm thinking somewhere along the lines of $45? Is that too much though? Too little?

Here is the work:
https://mail.google.com/mail/?ui=2&ik=555ad7bbbf&view=att&th=131afd6558b6577b&a ttid=0.1&disp=inline&realattid=f_gr57h4z60&zw

Thank you for your help.