I have one 12" that'll pound your ears
A 2002 12L7 Kicker Solo-Baric woofer in the Kicker ported box powered by a Rockford Fosgate bd500. (woofer is dual 4ohm wired at 2ohm, meaning its getting the whole 600-700Watts).
You don't even have to get a 12".. Get the Kicker 10L5 with the Kicker ported box powered by uhm Kicker KX600.1 (around 600Watts) if you get the L7 (handles a bit more power than the L5). If you get the 10" L5 - get the KX400.1 (600 watts might burn the 10" L5)... Assuming you run both setups at Dual 4 ohm for total 2 ohm resistance.
The 10L5 + Kicker ported box sells from my local shop for $270 and the amplifier for $250 (from eBay)... With cables/RCAs it should run you 550-560 the most, but the sound you would get is incredible...
Trust me on that.
This is the Kicker that I have (2002 12" L7)
The 10" looks the same but its 10" and the woofer is black. (looks like this
http://www.kicker.com/images/2002/products/S10L53_4R.jpg )
Soon2bXGrrl - don't bother with the factory speakers, get an amplifier and a subwoofer. Are you looking for SPL (loud) or SQ (quality)? If you're looking for quality you should get JL Audio - good priced very good quality subs. For base look into Kicker Solo-Baric. If you're tight on room get a sealed box (regardless of woofer), but if you got some room go ahead and get a ported one (its bigger than sealed, but louder).
A nice setup would be a JL Audio 12W3 in a sealed box with a Rockford Fosgate Power 250.2 amplifier (bridge it and it'll give you a nice 250-300Watts). That would cost you around 400 bucks with 100 for installation/cables/misc. It would sound good.
For base you could use a Rockford Fosgate Power 350S and a Kicker Solo-Baric 10" L5 woofer in a ported enclosure. The price would be a bit higher but the Kicker (louder) setup - but would sound much better.
Anyways, hope that somewhat helps and gives you ideas about pricing. If you got questions feel free to PM me.