Howdy.
Well, it depends. A pre-ban will cost you at least $1000 for a complete rifle of quality manufacture. A Colt will be at least $1200 (usually more) and a pre-ban Bushmaster will be around $1500, give or take. Generally speaking, if you are on a budget, go post ban. The only reason to go pre-ban, IMO, is the ability to use the telescoping stock and flash hider. A 20" rifle is better with a fixed A2 stock anyway, and doesn't really need a flash hider like a carbine does.
Now, a post ban Bushmaster or Armalite can be had for between $700 and $800, depending on where you buy it.
If you are really on a budget, you can build a "Frankingun" like I did.
I purchased a Rock River Arms complete lower for $190 and bought a used but excellent condition Bushmaster Shorty AK Upper for $385, giving me bascially the same rifle I could have bought from Bushmaster for $775 for $575. I already had AR mags, which are pretty cheap anyway.
There is plenty of AR stuff for sale on AR15.com. Check it out. Complete uppers can be had for even less than I paid there, it just depends on what is for sale.
Hope it helps,