no no no man. it's a stretch with only $500. more money the better but if thats all you have here's my suggestion...
for that slim of budget get a pa a3000db. one in the classifieds for 230. you will not find a better amp for $230 shipped.
http://www.caraudio.com/forum/showthread.php?t=332584
build a box yourself. double baffle the speaker mounting side, brace it good, and make sure it's your cuts are perfect. if you have access to someone experienced with tools have the inside edges rounded and 45 degree angle corner pieces. if you can get experienced fiberglassing done for cheap then look into it, otherwise you'll be spending alot more. You can get reasonable priced boxes built on here. best bet is local. supplies will be around $40-60 depending on box size and other variables. i recommend offering a round of beers or dinner to a friend who is good with buildign boxes and the 2 of you build it.
as far as the sub. pretty much any single spl oriented sub will work. i recommend a 15 for a balance of loudness and lows. Many things will be taken in account in the final sound, but if do things RIGHT you can make almost any size speaker do what you need. I like 15's. With limited power you'll wanna do a larger volume to help with efficency. By larger i mean on the larger side of whatever is within the recommended size specs. and go ported. tuning will depend on the sub.
Mt, BTL, mach 5 spl, treo ssx, anything spl, AQ
here's an hdc3 15 at the voice coil config to match that a3000db
http://www.caraudio.com/forum/showthread.php?t=332332
not sure of the VC but here's some shocker sigs
http://www.caraudio.com/forum/showthread.php?t=326364
btl 15 d2.......300 shipped. i love my btl, couldnt be happerier right now. shits stupid loud off this kx 2500.1 in 3cubs @ 33hz
http://www.caraudio.com/forum/showthread.php?t=327098
make sure you have 1/0, big 3, and a nice battery in the trunk for the amp at the bare minimum.
