I did a think with Youth Challenge International (
www.YCI.org) several years ago.
Great experience. The price was a bit much.
After airflight to South America it ended up being $4500-$5000. The money you pay in is a tax right off though. So it ended up costing me closer to $3000.00 (this was when taxes were higher).
It wasn't so bad considering I was down there for 10 weeks and that included air fare, accomodation, travel to and from work sites, food, buliding supplies, etc...
Since I did the program, YCI has expanded their program from South America and the Pacific Islands to include several sites in Africa.
How old are you? This program is really for anyone aged 18-30. If you are in the upper range or have lots of experience you can get hired on by them as a group leader. This is a longer commitment but they will actually pay you to go.