Peter C. DiCola has self-archived "Money from Music: Survey Evidence on Musicians' Revenue and Lessons About Copyright Incentives" in SSRN.
Here's an excerpt:
For most musicians, copyright does not provide much of a direct financial reward for what they are producing currently. The survey findings are instead consistent with a winner-take-all or superstar model in which copyright motivates musicians through the promise of large rewards in the future in the rare event of wide popularity. This conclusion is not unfamiliar, but this article is the first to support it with empirical evidence on musicians' revenue.