What's your size limit for the sub? You can build a sub that will compete performance wise with $500-$600 ID subs for under $300 total. A 20" wide x 20" high x 22" deep OD box made out of .75" material will net you right at 4ft3 volume before port/bracing/driver displacement are considered. This is a very good setup for a ported 12" sub. Take the following components
(use the full port length)
Installed in the above enclosure and you will be capable of the following
A window pane style brace should be used half way through the 22" dimension, with the port off set to pass through one of the cut outs in the brace. The baffle can also be doubled up, which will make the depth 22.75".
After you build a sub like this, and hear the level of performance you're getting for dirt cheap, you might consider building new speakers as well.