You would have to be more specific to receive an accurate price. The price all depends on which components of a sound system to want to replace and/or install. This can range anywhere from a couple hundred dollars to replace the head unit (in dash main stereo unit) to several thousands of dollars for high end speakers, custom boxes and mix of amplifiers all wired properly!
I have about $2500 into my stereo system of my truck and it sounds better than average with some major bass but no means a competition vehicle. Selecting the right components also help the sound quality.