Turtles are expensive man... at least the ones Ive seen.
I would suggest you look into tortoises. Sulcatas get very big but are interesting and rewarding to keep.
Box turtles usually hover around 100$... I think lllreptile has some.
And here are some of the commonly kept tortoises, may get a bit big for a 20l though.
http://www.reptiledepot.com/torts.html
The red foots would do okay in a 4-5 foot enclosure i think.