Selling Copyright?

[vibris-sae.livejournal.com]

I was recently commissioned to design a logo for a roller derby team. However, I have been asked to sell the copyright. I don't mind doing so, but I am pretty naive about this sort of thing.

The team is pretty amateur, but they plan on merchandising with t-shirts. The commission fee is $100. What should I charge for the rights?

Thanks a_b!