A sample purchase of 10 rock, folk, country, and classical albums including six recent releases and four back catalog showed a distinct difference in the purchase price of tracks.
The comparison is particularly interesting since iTunes owns Lala.
SELECTION - According eMarketer in January of this year, iTunes had roughly 11 million tracks, Amazon sold 10 million and Lala offered 8 million. Rhapsody, eMusic, and the Zune Marketplace each had 6 million tracks to choose from. (via ZDNet)