I'm very out of touch with the Car Audio scene these days, but the available options are generally of the kind manniraj posted. I used to use a similar (much older) Blaupunkt model many many moons back, and it did nothing to improve the bass really. Especially it was not enough bass for the buck. I changed it in a few months and went for a proper amp + sub combo.
Getting a separate sub + amp would definitely work out to more than 10K, so depending on your usage, you might want to settle for an under-seat sub.
I've seen a very slim (8" I think) sub-in-an-enclosure from Kicker. It is mostly SPL oriented, is not under-seat sized, and will need a separate amp, but it was a fairly decent bang for the buck proposition considering the bass support.
For somewhat higher budgets, if space is at a premium, I think there are pretty good 6.5" subwoofers from HAT audio, and 8" subwoofers from other good brands. The boxes will be fairly small and will not need a lot of space. You can also have the boxes made removable with speakon connectors. If you choose something that works with a sealed box, the box can be quite small. If the amplifier is kept under the seat, you won't lose a lot of space.
Alternatively (even more expensive) there's the option of fiberglassing an enclosure into the spare wheel area in the boot. In most cars you can get away WITH the spare wheel inside. But in this case you might need a 10" sub. Also, removing the spare wheel will be a bit fiddly.
EDIT:
http://www.crutchfield.com/S-YZhJ7UXa0BC/p_108BASSLIN/Infinity-BassLink.html The Basslink used to be a very popular compact active sub. It looks like it is still available. More or less within your budget. Do check with a JBL Store if they have it here. There also used to be a similar option from Kenwood (called WooX I think).