i believe that the best value out there in automotive subwoofers is the
elemental designs k series. for a cheep price, you get an efficient sub that will give you awesome sound quality while still retaining the ability to get very loud. on top of all that, they perforum very well in either
sealed or ported enclosures. now that link gives a very basic (and overgeneralized) explaination on each type. if you are interested, i could direct you to some fine reading about ported enclosures.
hell, you could get two of them subs and a
JBL bp1200.1, which is just inside of of your listed budget. that would prolly be a lot more bas than what you need

Tempe