I shoot a Mathews Outback, one of the high priced bows. Whether its Mathews, Bowtech, or Hoyt, their top of the line bows are expensive. Some of the reasons have already been mentioned, machined vs. cast, bearings instead of bushings, ect, ect. Now, sound travels at somewhere between 1000 and 1200 fps. No matter how fast your bow is, it's slower than that! So you better be thinking quiet. Quiet comes with smoothness, a little heavier arrow and all the other little things that goes into a bow.

That being said, I know a guy that still kills a doe or two with a compound he bought 25 years ago. (He wouldn't take that bow on a trophy hunt though)

And finally, why do I buy the best? Because I can.