pioneer headunit, probably like a 3200ub or something around that range $60, look at used headunits and you will stay in bugdet, i like pioneers because of the ipod interface and they are decent for the price.
sub - 12 in dcon in a ported box, will get loud and has great sq- $150 including ship
box- learn to build it yourself, you will stay in budget and learn a great skill $45
Wire - KCA Complete 4 Gauge Amplifier Installation Kit $26, good wires and you can use them later when you start upgrading
Amp - hmm thats tricky, you will want something with around 400 rms or more at 2ohms, look at a memphis 1000d, or if you dont care, im going to get **** for this, but an audiopipe 1000d would probably be your cheapest route with this setup, not the best amp out there, but will do the job for ya. around $100
if you can do it right your looking at like $350 for everything, but if you can get some deals and look around you can cut some costs with the box and amp. good luck. o the dcon is an amazing sub, i own it, it will handle more than twice its rated power and sounds great on it, they are made really well, and in the usa, so it will be a quality product!