I'm just throwing this out there, because i'd love for this to work out. I know this is unasked for, and you are more then welcome to ignore my questions/suggestions, but I would feel remiss if i didn't do this. If I were looking to do this, I would get a feeling of how many people will attend, by going to the strange assembly website and the AEG forums to measure interest. Also what kind of venue are you hosting this at? What kind of attendence can it handle? Have you thought about trying to do an online tournament

? More of a headache on your part, but its a cheaper venue.
Finally, I would contact Todd at AEG, or another AEG representative, and see if there is a way you can get some sort of arrangement for prize support revolving around them. They could do a whole host of different things, and who knows you may end up arranging what would be the next chicago kotei in terms of helping out the charities!

P.S. I would also set the entry fee higher as well. Once you get the prizes determined, I'd quantify the prize costs, for instance, if you determine you want to give the whole box away to first place, then a 20 dollar entry fee isn't that bad. Thats how slot machines work, right?

I mean technically we are gambling in a sense. I'm just saying "asking for donations" will ultimately get less money overall, because people will naturally skimp on their donations. Its human nature. So if you can raise the minimum, then that would be better for the charity.
Let me know if you need help doing any of this, I can volunteer some time to help.