Good evidence of commercial impact, and the damage of orphan works:
Heald, Paul J. and Erickson, Kris and Kretschmer, Martin, "The Valuation of Unprotected Works: A Case Study of Public Domain Photographs on Wikipedia" (February 4, 2015). DOI:10.2139/ssrn.2560572 Available at SSRN: http://ssrn.com/abstract=2560572
Michael