I put one together for under $120. Mind you I wasn't looking to rattle the dishes, just more punch than the stocker. I mounted the subwoofer and amp to a hardwood cover I made to help eliminate noise from the trunk pass through. I can quickly pull the fuse, disconnect the amp and remove the whole thing before going to have little high RPM fun.
It is low budget, but simple, sounds good, and easily removable. If this is what you're interested and can take some pictures.
that is sort of like asking how much torque do we need?
there is no definitive answer. But a guideline is twin 8" subs give better bass for Jazz than a single 12"+ would (in most cases). For hip hop, dual 12" subs with dual voice coils and a rattling license plate are favored.
that having been said, most people are very happy with a single 10. There are ton of ways to do it right. It boils down to a couple of factors... ease of install, cost and look/space occupied.
Boxes are easy to install and give good bang for the buck but look like shiat. free air subs (speaker attached to board) are easily damaged and look like (see above). Self-enclosed subs look good and are usually priced well, but don't usually sound that great.
This is why I posted about my great experience with that Infinity sub. My car before the current one, an E36 M3, had over $12k of sound mods. A combo of pro build and self-installed, but the system was a/d/s amped, a/d/s aluminum dual coil 10" dual sub, dynaudio drivers and sony mobile ES digital time-domain shifting equalization. Yep I was one of them freaks with a laptop and sampler mike in the car tuning the sound. Now nothing will sound like that car, but this Infinity is amazing for the cash.