Not to be objective here, but, although the "Ch" looks a tad "unusual," the rest has a definite air of authenticity to it. Of course, Buk's signature on an otherwise unassociated end paper (that's what it appears to be), is worth perhaps $50. Well, that's my opinion. Buk's sig varies in value depending on what publication it is in. In other words, the difference between the trade hardcover and a signed edition varies considerably from, say, Post Office to Betting on the Muse. All else being equal, forgetting seriagraph prints, original artwork, signatures with little man or little dog or big bird drawings, this siggie looks to me to be worth no more than $60. If real, which I'm inclined to think that it is.
Hell, I'm no expert, but owning some dozen or so signed Buk things, and spending rainy Saturday afternoons sifting through the signed manuscript section of this site, I would consider it genuine. Ugly and hastily signed, but genuine. It's apparently a cut-out end paper. If thrust in front of Buk, he would have likely signed it quickly. No COA would make me feel any better about that, though. I'd trust my eye more than anyone else's.