Got one of these in my M3, Im not really into car audio, 10" sub with built in amp and crossover etc.
Its very good as new condition excellent installation that doesnt take too much of the boot space,
look very high quality I think they're US$600 new.
$500 Waikato/Auckland 0221002616
http://www.bavsound.com/BMW-Underseat-Subwoofer-Upgrades/d110.e46-Ghost-Subwoofer-System/Gallery/