I did this at my nieces' brownie meeting. Here's how it went:
Each parent paid $5 for the craft. There were 10 girls I think. (So $50 for a couple hours of work) We made two cards. One for Valentines and one for a Cookie thank you. Parents helped with the stamping. I brought glue sticks instead of tape runners. One of the troop leaders bought some supplies for future crafts and another mom placed a pretty big order, and I applied my $50 towards the workshop since she's a friend and I wanted more ribbon!
All in all, I would do it again. I really had a good time. The girls had a great time stamping. I wouldn't do too much differently, except maybe make the projects a little simpler and not have a project that requires coloring in (it took way too long). There were many moms interested in coming to workshops if I had them, so I would have a sign up list or something like that.
Oh, and one more thing. Try to make sure you have someone there who has stamped before. They can help with one project while you help with the other.
Hope that helps!