For about $400 I'm going to be putting a Dayton Audio Titanic MKII in for about $150.00 from www.partsexpress.com. For the $250 left over for the amp I'd look at JL Audio stuff or a JBL Powervalve amp. You can get a JL Audio 250 watt subwoofer amp for $309. from www.onlinecarstereo.com or you could get a JBL Powervalve BP600.1 for $253.95 from www.mmexpress.com
Let me know if I can help you with anything else.
P.S. There are always good deals on ebay also, but beware of who you buy it from.