This is the hardest part about catering--figuring out what to charge.
I think it is easier to think in terms of days, rather than hours.
For instance, you'll spend at least a day prepping, and then the party, with the before-and-after work, becomes another very full day. What's two days of your time worth to you?
If you're anxious for the experience, you may let them pay you whatever they feel it is worth, just to get your feet wet. On the other hand, if you're very busy, you want to be sure to be compensated enough.
Whatever you do, be sure you and your friends have settled this beforehand. They may have no idea how much work it is to create good food, and you don't want any hard feelings.