the stock sub costs a lot because IT IS FROM THE DEALER. hell, BOLTS cost $8 from a dealer.
Odds are you can find MANY people selling stock sub woofers. I know I sold mine not too long ago. I think I asked for $20 for it. You could have also blown the amp for it. If you need a new stock sub amp, let me know I'll give it to you for $15.
To get at it, use a size 50 torx(sears or home depot have them) socket to take off the rear passenger side seat belt (the top part). Then use a socket (7mm or something) to take out the hanger hook above the door. Then just start near the cargo door and pull. Everything will snap out. There might be a few screws at the base of the big plastic panel where it meets the carpet (pull the carpet back a tiny bit and look along the end of the plastic panel). You'll also have to slightly pull up the plastic piece on the floor near the door so that the panel will come all the way out. IT IS SIMPLE - dont waste $$ having someone else do it.
If you know how to undo screws and plug a few wires in, you should be able to fix your problem for < $30. If you spend more then that, I will come to NY and slap you =P... unless you upgrade your system.
Any Q's you can pm me if ya want.
Edit: Incase it isn't obvious to you... fold down the seat before you try and take the panel out....