iTunes plans to implement a new variable pricing structure based on supply and demand:

True to supply-and-demand economics, the price of music downloads will be geared to the artist’s popularity. Releases from new artists would receive the lower pricing, while tracks from popular acts would get slapped with the higher rate. Even classics, such as Bruce Springsteen’s “Born in the USA,” could retail for the higher price. Most of the 10 million songs in the iTunes catalog are expected to remain at 99 cents.