Getting a car audio system?

i'll be getting a used car in about a month or two, and one of my biggest priorities is getting a good audio system in it. now i'm not talking about an audio system that's super loud and pisses everyone off... i just want to install an aftermarket stereo (the front piece), get the four original speakers replaced, and maybe get one sub-woofer to make sure the bass sounds good. i'm guessing that the stereo, speakers and sub-woofer will cost about $300-$400 for good ones. my question is how much will it cost to get all of it installed? and there are only two places around me that offer installation... best buy and a family owned shop called PJ's Auto Sound. i just want an estimate on how much money they will charge to put all the parts in. any advice would be appreciated!