I have tried to stay away from these events because you run a huge risk of not covering your time and costs if no one buys. That said I actually just did a week long shoot for prints for a kids gym. Now this was not an event per-say, rather 27 classes of gymnasts in total about 700 kids.
They couldn't guarantee me anything, only that in the past a good number of people had purchased photos. I crunched my numbers and felt my worst case breakeven was below what I expected to shoot, and my expected profit would be above my base rate, so for the first time I did a prints only shoot.
End of the week, my sales were almost directly in line with my projections, and I ended up making my normal hourly rate and profit. On top of that I got another large shoot out of the event for another club.
I guess my point is, you can make money on these, however you need to do some research and determine what the potential revenue opportunity. (high side, low side, expected), then factor in you time and effort as well as costs and see if you believe that you can profit from the shoot. Without any information on past years sales, I would have passed at this one without a second thought, but now I am glad I didn't.