If you want the "latest and greatest" ball, then you will usually pay about the same on every site.
If you are willing to use the previous hot ball instead of the current flavor of the month, then you can find them at a decent price. Check the listings here, or Ebay and other online auctions. Not every brand will be discounted. Storm for instance, rarely goes down much in price even after the ball has been discontinued. Brunswick, Ebonite, Columbia 300 and 900 Global/AMF will often go for fire sale prices within a year of the original release for most balls.
Buddies Pro Shop has some really good prices on closeout stuff from time to time. You can sign up for their newsletter on their site.