If you want to spend a little bit more and get plenty of sound, go with the JL stealthbox. It's $750 for the box with sub, but it looks like a stock piece of equipment and sounds like 2 12's. Plus it's fiberglass so it's more durable and lighter than mdf. And you still have all the storage space behind your seats for jacks, wrenches, etc.
I have that with a 500w monoblock JL amp, and it sounds awesome. It's not even turned up all the way and I don't go more than halfway in volume on my stock stereo.