I purchase the deck used (2 months old) and got the speakers and amp very reasonably. I think the total including installation was just over $1,200. I did not however go with any subwoofers, just the 6"x9" rear speakers, 6-1/2" door speakers w/ separate tweeters in the dash. I am quite pleased with the sound upgrade as I has decent bottom end punch.
If I was to add a subwoofer or 2, I would put them install them in the rear storage compartments. I have seen this done in a few C4's and it looked quite good and the storage of the 2 trays is very minimal at best.