According to the abstract of this 30-page paper:
This Kat is no economist and, though he generally knows when to nod in agreement or shake his head in disagreement when talking with those whose educational and professional background include mathematics, he must confess that he would find it hard to pass an informed comment on the methodology that led the authors of this paper to reach their $246 to $270 million dollar figure. If any readers have comments to make, or light to shed, on this curious piece, he will be delighted to hear from them."What is the value of works in the public domain? 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 most historically remote subjects are more likely to have images on their web pages because their biographical life-spans pre-date the existence of in-copyright imagery. 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.
To get an idea of some of Paul's other writings, you can access his SSRN author page here.
Wikipedia's Public Domain Image Resources page here