If you've got a fairly modern car, spend all the money on a H/U and run it through standard speakers, you'll get far better clarity. Cheap sub's are false economy because they sound shit and you look a twat when you're sat at the lights banging loose, distorted bass out, not a good look. Like jake said, if you wanna go budget, spend £50 on a half decent set of 6x9's and stick them in the parcel shelf (level...) and you'll be a lot better off than buying a shit sub/box/amp. Just to put into perspective the cost of decent gear, my component (door) speakers cost nearly twice what you want to pay for a H/U and sub, £385 for two mids, two tweeters and 2 crossovers. Edit: If you've got an amp already, run the 6x9's off it...