The ASW610 by B&W is cheap (~600 bucks) and will blow your face off with its lows and compact size. Rubberized paint is cool too.
B&W CM series subs are great too, but now we're talking big bucks.
Polks psw505 12" sub isnt all that bad too. lows could be cleaner though.
Sunfire makes a mean sub too. not sure about price.