Well, by "dirt cheap" I mean that, with a current daily purchase rate of 25 BTC, each note is priced at approximately (25*$350)/5,000,000 = $0.00175. This values the total Note supply at about $2.6 million.
Compare this with the valuations of traditional music services like Spotify, Pandora, Beats, etc. (which provide less functionality than Peertracks/Notes will be able to), and then you'll begin to get some idea of the potential...