Full auto? there's mac's floating around, decently affordable.
Semi? since its not going to be practical, a just for fun sort of thing, I would go with an AB-10 (tec9). Accuracy sucks and not reliable, but if youre just looking for a gangster range toy, go gangster!
something more practical? the Beretta Cx4 line gets rave reviews. $835 on buds though.
The Hi Point carbines are a reasonable choice. Nice and cheap too. $300 max? something like that.
The Kel Tec one folds in half...thats pretty neat.