It appears that the best option for your price includes this list of hardware:
AE AV15 15" Subwoofer Driver ($250)
Behringer EP2500 amplifier ($280-$350)
Behringer DCX2496 DSP xover/equalizer/processor ($275)
After the hardware total, you still have enough in your budget for the wood and finishing materials to build the cabinet.
In the proper slot ported cabinet, you should be able to easily produce 123-125 db from 17 Hz on up in an average sized closed room, with the subwoofer corner loaded.
You may wish to exchange the OEM fan in the amplifier. It is a bit too loud for most people. It is a cheap and easy change. It is the same type of fan as used in a computer case, and is about as complex(that is, it's very easy) to change out as a computer case fan. There is a very specific model fan that is the right voltage (24VDC) and is silent that you should use for this application.
If you are interested in this suggestion, and want specific suggested enclosure parameters for the above driver, let me know.
-Chris