WI governor proposes state sales tax on downloads

Now here is a silly proposal. My question becomes how does a person value a 5% tax on something intangible like a download. Does one make up a retail price for Edith Nesbit's The Railway Children even though it is free and in the public domain? Or how about something like Cory Doctorow's Down And Out In The Magic Kingdom? Do I go with the Amazon's price or the retail price? Doesn't this violate the Creative Commons copyright in some sense?