Whatever you do, DON'T buy those ones you see in mail-order catalogs.  They are terrible-sounding, terrible to play.
Musical instruments are one part manufacturing and 3 parts skill.  We know exactly how violins are made, yet no one can figure out how to duplicate the sound of Stradivarius violins exactly....it's never the same.
The bottom line, expect to blow a few-hundred clams.  If you've played before and know you're going to continue playing, it wouldn't hurt to blow $500-600 and get something a little higher-end.  Don't ignore the used market, either.
If you've never played before, you might want to look into a rental plan, just to see if you think you're going to stick to it.  Many places will do a rental plan where if you decide to buy, your rental payments go at least in part towards the final purchase... 
EDIT: For $500, don't expect to get anything NEAR a great model.  If you want high-end tone and craftmanship, you need to be prepared to spend a couple thousand.  $500 will score you a good-enough starter/student model, maybe a used but good-quality instrument.  Musical instruments just ain't cheap.