Well if you figure they are charging you 50.00 for the install, then you are spending 300.00 on the amp/subs. I think you can do much better than that setup for 300.00. Have you thought about used equiptment or does it have to be new? You can get a jbl p180.2 for 139.99 which will supply 360 watts to a 4 ohm load. That should be plenty of power for one sub. Now you have roughly 150.00 left for a sub and box, that should be enough for something fairly good. I'll try to find some examples in a bit.
http://www.etronics.com/product.asp?stk_code=jblp1802&store=&rpc=4047&catid=306 
