By the way, if your amp has a mono mode, you could use your fosgate 250 go to a single 10" and it will sound great if the 10 is in a properly ported enclosure. Then you could by another Fosgate 250 and another 10" sub/enclosure and do the same thing.
The result, use the amp you have, buy 2 10" subs and build 2 single 10" enclosures.
Price varies on subs and enclosures, but if you go with the bazooka rs idea, the 10" bazooka rs non-amplified sub/enclosure is about 150 dollars online. So you could go with 2 bazookas at 300 dollars plus and extra Fosgate 250 at however much they are selling for. Make sure you use a crossover to filter out the highs and mids from the subs.
Good Luck