e-songs sell much higher volume. Therefore, e-books must cost more to cover the fixed costs (someone else posted a breakdown of book-costs in an earlier thread).
Additionally, the effort required to produce a song is much less. I could write, record, mix, and master a song in a week. A book would take several months just to get a first draft.
An album of songs is much closer in terms of production costs, and requires a similarly large effort.
Edit: http://www.tobiasbuckell.com/2010/01/31/why-my-books-are-no-...