I thought they were cool 20 years ago too. I literally have a short box full. Then I realized they manufacture their own exclusivity and hype. Very VERY few if any of their books have ever held value let alone increased. And if they did increase it was the book itself and not DF Certificate that had anything to do with it. I found 3 of their signed books that were limited to 75 copies in the $3 boxes at my LCS just 2 weeks ago. PLUS, they started marketing CGC Books. I found out the hard way they were not CGC Exclusive or DF Variants, but just regular old A covers of a book. So basically $80 to get a 9.6 of a book on pre-order. In most cases the books aren't worth $80 in 9.8.
Rant over. If you see them for $5 or less go ahead and nab them. Just remember some poor schmuck paid a minimum of $20 for it at some point. I don't remember seeing any DF release under $20.