I just installed a factory sub box with a 8" CVT powered by an Alpine 200w amp. That thing rocks! I never expected so much bass from an 8" sub. I hits harder and carries further than a single CVR 10" sub I had a few years back. Amazing sound if you listen to Rock/Country. I am very pleased with the setup.

Most bang for your buck eh? Start here, a single 10" woofer, box, and an amplifier to run it. That will be about the most bang you can get while spending the least. Other options will give you more bang, but for more buck. Here are some options, starting from the most inexpensive with the least bass to the most expensive with the mostb ass.

Single 10" with a 400Wrms amp

Single 12" with a 500Wrms amp

Dual 10" with an 800Wrms amp

Dual 12" with a 1000Wrms amp

Cost will come from having more than 1 subwoofer, certain subwoofer brands, certain amplifier brands, more powerful amplifiers (which also increases cost due to installation wiring), enclosure type (a larger box for two 12's will cost more than a single box for a 10"), and purchase locations. Honestly, if you're looking for bass on a budget, get an Alpine MRP-M350 amplifier, a shallow 12" Pioneer, a single 12" box, and 8awg wiring all from ebay. I'd almost bet you could get away with spending $300 or less.