I'd agree that the Sundown SAX-100.2 and SD-1 12" sub combo is a great deal then. Only thing is, you'll have to spend a little extra for wiring and box materials.
The 10" sub combo might be more economical for you since they can be placed into a smaller enclosure, which could save money on box materials.
Another option if you are willing to do a single sub setup:
Assassin 12" $119 shipped (underrated... will take a lot more than 250w rms)