We gave them one 8x10print framed to 11x14 plus an Cd labeled Help-Protrait 2010 with up to ten out takes from the shoot all on site we did 160 people had 5 sets going all worked out great. Ed
Yeah, collecting donations is miserable work. But how much money do you need for your project? If you've got twenty volunteers, can't you get each of them time to kick in a few bucks and cover all your costs?
I did one of my solo shoots last year for sixteen families at a local food bank. It cost me less than $50 (total) to give each family one 8x10 and one set of four wallet-sixed photos (via Costco).
The other option is to consider asking some of your volunteers (who have jobs) to ask their employers if they have any sort of charitable giving programs.