well they're not common, I've seen a couple and heard of a couple others, and if they were only on the GMC big trucks, there won't be a lot left, but I'd say worth about the same as any other TF cab, which depends on condition .... I've always thought they were kewl, but don't know I'd actually use one, and you'd definitely need to make sure you got good corner glass with it .... also, make sure it is a Chevy dash, it's no fun changing out dashes!
Bill