I saw rackshack.net advertising a 1.8GHZ Athlon with 1GB memory and a 40GB disk with zero setup charge for $100 US per month a couple of weeks back. It's not $20 per month, but neither is it $300. You might want to use such a machine for the sites you already run, and charge this non profit $50 for hosting.
If you want the group to go with ACS it sounds like you'll have to show them how easy it is by demonstrating the sites you've already built, and put to rest their fears that it will be too much work for volounteers (i.e. them) because somebody who knows better (i.e. you) will take care of it.
Good luck.
