I paid $30 for mine but they really range in price depending on what type. They also range in size from 3-12" or so. I think a smaller one would be fine in a 6 gallon as they don't really move around much. As a juvenille I would suggest feeding every 3-4 times a week and 2-3 times a week as an adult. With that small of a tank I'd keep an eye on water quality but since they are inactive and eat very little it shouldn't be a problem. They also excret small pellet like waster so it's easy to spot and clean up.
I feed mine a piece of freeze dried krill soaked in vitamins about 3 times a week. Just make sure the piece isn't too large, I suggest reading the thread on RC, I know it's long but goes over a lot of important points on feeding them as they will pretty much eat until they die if you let them. Food will basically rot in their stomach faster than it can be digested hence the reason to feed them smaller pieces on only every few days.
I do not keep anything else in my tank but I think I will once I find a suitable tankmate?
(i.e. something too large for him to eat but not too aggressive that it will pick on him or will outgrow a 30g tank