I have heard good things about the Basslink unit and they go for around $200 on eBay. I haven't had personal experience with one, but I have had both the Subaru unit (it's got a super small foot print, is self-powered, and fits underneath either of the front seats) and currently I have a Kicker PC12 self powered unit in the trunk. The Subaru unit was neat because it was so small and produced just enough bass to make a positive difference. The Kicker unit I have is pretty good also, but is huge. It takes up about a quarter of my trunk space.

Alex