I've done a few sittings like this, and I too refuse to take any money beyond my costs. Had I had a professional portrait service, all would be different of course.
It's fun to do, good practice, and if you pay attention and do a good job, it's rewarding too.
I make them pay for film, chemistry, water, paper, electricity, my time, presentation materials, CDRs, albums, etc. All of it. Then I make it up to them to throw in some tip, if they're happy.
I have an event like this coming up this weekend. Two rolls of Portra 800 120 and one roll of Portra 160 35mm, processing, scanning, CDs, prints, etc - I make the lab scan all of the frames, and then I pick the five I'm most happy with and make high res scans at home. Depending on what you charge for your time, and how much you want to get in return on your investment in software, computers, scanners, darkroom supplies and equipment, etc, it can vary from 70-80 up to several hundred dollars what your cost is. It matters whether you have an Epson V700 scanner or an Imacon Flextight, for example. Or if you have a Pentax K1000 or a Leica M7 with a $5k lens on it. Etc.
Establishing exactly how much you spend in materials and time will help you a lot in setting a fee for your services.