As far as my car, I had 2 12's, but they finally gave up on life, so I decided to go with a single 15". I built the box myself out of 3/4" MDF board to the volume the woofer needed.
I was on a budget so I got a 15" Boss (cant remember the style/name of it) but it was 60dollars new on ebay, and I really do like it. As far as trunk room, there's plenty in mine, because the box fits right down there the spare tire used to be.
At first there was quite a bit of rattle, but with some foam tape, theres no rattle at all anywhere.
It pounds very hard, and all the properties on my Sub (2000watt, 2 channel, bridged) are turned all the way down (bass boost, etc.)
And as far as cost, since I already had the Amp, wiring, and the MDF board, it ran right around $80dollars, $60 for the sub shipped, and $20 for glue and carpet.
