OK, so is there a mean price? what should I expect to pay to get decent sound?
Oh, boy... that's sort of like asking "How high is up?"
So much depends on what level of (or rather, lack of) audio quality you will be comfortable with.
My recommendation for someone who wanted his car to sound really good, but not go totally nuts and build a competition quality system, would be:
1) Replace the factory speakers with some good quality speakers -- figure about $150 per pair.
2) Add a subwoofer to the system. This is absolutely critical. If you don't do this, then don't bother doing any of the other things on this list either! Figure about $200 for a decent quality single 10" subwoofer and sealed box.
3) Add a good quality four or five channel amplifier to the mix. I won't go into brand names, we try to avoid discussing religion on the bbs. But you're going to have to spend about $250 for a good amp -- somewhere around 250 watts is plenty. You don't need to have kilowatts of power in your car.
4) Be suspicious of any stereo shop that offers to install all this equipment for less than about $100 -- unless you bought the equipment from them in the first place, then they may give you a price break on the installation.
So, we're looking at about $850 in addition to the cost of the player. This will give you very good sound. Lesser expenditure will give you lesser sound. It just depends on what you can live with.
That said... you can cut that price about in half (or better) if you know what you are looking for and are comfortable shopping on ebay. But if you lack experience in the car stereo arena and/or with ebay, don't do this without help.
tanstaafl.
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"There Ain't No Such Thing As A Free Lunch"