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 

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