We have been to a girl scout event, where we brought our own meat, and we were fine with it, but a few parents didn't bring anything, and of course we felt we had to share(luckily we were raised always bring a little extra just in case, as was another family).
I would suggest probably supply the hamburgers and hot dogs, and have them bring their own beer, since beer can but even more $ then the meat. I agree, I would go to Costco. Around here, Ball Park beef franks are 7.99 for 3 packs(at the reg store they are 4.99 per pack). Don't remember the exact package size or price for hamburgers, think it was like 13.99 for 18 of their sirloin patties. Reg hamburger meat ones are even cheaper.
We also have a Smart and Final around here, don't have to have a membership, and still has great prices on bulk products.