Paul J. Heald et al. have self-archived "The Valuation of Unprotected Works: A Case Study of Public Domain Photographs on Wikipedia."
Here's an excerpt:
We study the biographical Wikipedia pages of a large data set of authors, composers, and lyricists to determine whether the public domain status of available images leads to a higher rate of inclusion of illustrated supplementary material and whether such inclusion increases visitorship to individual pages. We attempt to objectively place a value on the body of public domain photographs and illustrations which are used in this global resource. . . . We find that the large majority of photos and illustrations used on subject pages were obtained from the public domain, and we estimate their value in terms of costs saved to Wikipedia page builders and in terms of increased traffic corresponding to the inclusion of an image. Then, extrapolating from the characteristics of a random sample of a further 300 Wikipedia pages, we estimate a total value of public domain photographs on Wikipedia of between $246 to $270 million dollars per year.