I just got my subs hooked up a couple months ago. 2 12" Sony Xplods. Sounds pretty sick. Got matching Xplod deck and amp.
I was gifted the amp and subs though, and it still cost me $300 once it was all done.
Hope you know what you are getting into before you get started.
Deck costs $100+
Wiring all in will be about $75
Harness, Antenna Adapter, Deck fitting kit,
and installation if you get someone else to do it.
Plus however much you pay for your amp and subs, could be another 400.
good luck though, im really happy with mine: