The answer to the question depends the system. If you want to seriously compent in spl competitions then you should prolly go with a expensive high powered subwoofer that going to require a large expensive amp. If you just want some bass for your daily driver then you can get a good system from sonicelectronix.com for under $500 bucks. Here is an example.
http://www.sonicelectronix.com/item_5388.html Now I'm not sugguesting you buy that system, but I just wanted to show that you can get to 2 good twelves and and amp without spending a lot of money.