From a budget standpoint, the cheapest way to get the most gains/$$$ would be to replace the ridiculously restrictive 6-cat H-Pipe with an Off-Road H/X Pipe.
You can leave the stock mufflers (which don't flow badly for stock applications) and dump the midpipe and recognize the most gains within your budget. I did this, though I spent $400 on a catted H-pipe. In hindsight, I probably should have gone with the off-road H and saved $200-250, but I didn't want to mess with MIL eliminators (to avoid a check engine light from the lack of cats.)
You can weld in mufflers if you want to change the tone of your exhaust. Depending on the mufflers you buy and the shop that does the work, it may be wiser to spend a little more and install catback exhaust yourself (if you're capable of doing so.) Do this for the looks/sound, not for the performance gains (basically negligible in near stock applications.)
Other people may be able to chime in with affordable catback system, if thats what you're going for. The best bang for the buck, however, is to focus on the midpipe.