o woops, i thought you said under 50 dollars, well than you can do A LOT, i personally did a 20g high tank because i dont want to venture out into bigger tanks until i can get a small tank down and running.
in total my spending was about 100 bucks for the tank + filter + top + heater, and the gravel was free, DW + thermometer + dechlor was about another 34 bucks or so, but it all depends on you really. also you have to account that if you want to have real plants, you need to buy lighting and thats pretty much the most expensive part of your tank if go in that direction ( thats what i have been told )
what austinsdad said was true about 55g being a decent sized tank for newbies. i can tell you from my own experience that, in a 20g things change pretty fast so you need to be on top of it ^_^
live for the moment, cause you will never know if you will have a future...