http://caraudiosecurity.com/shop/pro...s_id/8047.html is a nice setup, about £150+P&P
i have this setup and there is more than enough bass

but like i said for the best results from any sub you would need a custom box take a look at liam_b's setup here..
http://img140.imageshack.us/img140/3011/img6607nh6.jpg
stick to quality brands and look around.