There are a lot of factors that can be looked at in hosting a charity game:
1) Fields will generally waive entry down to the cost of the insurance cost only as a donation as long as the charity will provide them with a receipt.
2) Any donation over $20.00 should come with a receipt for the donor (tax purposes)
3) Manufacturers/Retailers can generally provide product for a raffle to raise money, and again the receipt for it does wonders for getting good stuff.
It's an incredible ammount of work, but when you're done, the reward can be staggering. I've seen events raise $5000 - $20000 in a single day of paintball play for charity. If paintball can do that, airsoft certainly can.
The Tier One Tactical Operator
Tippmann Arms M4A1
KWA USP .45, KJW P226, KJW hi-cappa