Congratulations on the find. As others have stated it can be hit or miss with value. Some do not care for the error / double covers, but that said there definitely is a market and keys with them always seem to bring a premium. Similar to Mark Jeweler's inserts. There are books that are known to have some double covers and are a little more common. Others are a lot more rare. It can also depend on the condition of each cover . One of the few down sides to these is usually the nicer of the two covers is not the visible one , so they present with less eye appeal. I have about 30 or so different double covers myself from a wide variety of books. They are awesome to find in the wild. I have also found other errors such a blank panels/pages, siamese pages/wraps, out of order wraps, print errors/missing colors, etc. Some of these fascinate me as the quality control should have caught them and they shouldn't have made it out. Unfortunately CGC doesn't have a separate category or way to truly see how many have been graded. This coupled with the same reason you found yours ( some never open the book to even know) it is near impossible to know how many exist. Anyway congratulations again on this find. If you would please share when you get it back from CGC.