I am not sure off hand what percentage of the value of the public domain is realized with a creative commons or GFDL license, but I think that it is less than half with even the most liberal creative commons license, the attribution one. Creative commons has many levels of usage requiring the re-user to tell who the original author was, restricting the user from making money by using a work, requiring the user to re-share what they have made and even restricting the user to change the original work in any way.
It is clear that if the non-commercial creative commons license is applied, the re-user cannot even sell a derivative work. The value of that potential sale would have to be factored into the percentage value of creative commons vs. public domain. But just the simple fact that a re-user must attribute a work to the original author is enough to stop many from using the work. The derivative work may need to be formatted in such a way that does not allow attribution for aesthetic reasons. Placing works in the public domain does not preclude a re-user from doing anything she wants with the original work.
Take a look at this video in which I combined 2 videos in the public domain with the soundtrack from a public domain song and my own voice to teach about setting up a blog for use in a classroom (some of you may have seen my other video Rick Noblenski at OpenEd 2007, this is the other one about blogs).
Our project group cited the sources from our videos at www.wikiblogedu.org out of courtesy but in this case they are all in the public domain so I don't have to worry about attribution etc. One of the original videos teaches about vandalism and encourages youth not to vandalize. The other video is of a career counselor who meets with a student who no longer wants to sell shoes for a living. The student has dreams of being an architect but the counselor effectively shoots down those dreams and helps the student to be happy amounting to less in life and continue to sell shoes. If a no derivative works license was put on any of the elements of this video, then the video would not have been possible to make (perish the thought!).
OERs are different though. I don't believe that their value is less than half of the public domain because most of them have instructional value and are somewhat polished. Putting them in the public domain may not result in a significant increase in value except in the areas mentioned above.
I find it interesting that a mathematical calculation can tell us the optimal time to allow a monopoly on created intellectual property. Especially since the calculated times given are much shorter than those currently in place in the United States. I feel like our copyright laws have gotten far out of hand. Especially when you hear of people saying that copyright should be “forever minus a day.” Ridiculous.