iTunes have been one of the few pay for download services that have been very willing to release figures about sales since day 1. Any time anyone has asked for download figures they have been more than forthcoming with regard to weekly sales. It is easy to accumulate these figures and come up with a sales pattern for iTunes.
In the first day April 28th iTunes sales were at 200,000 per day. By May 5th CNet were reporting that sales had topped 1,000,000 meaning 140,000 songs were been sold per day. By May 14th this figure had fallen to 125,000. While figures published in the The NY Times on May 28th translate the figure into 100,000 per day. The decline continued from there. 5 million tracks had been sold by June 23rd meaning the average daily sales had now hit 89,000. The figure hit 6.5 million on July 22nd translating into 52,000 sales per day.
News source: Slyck News