Kat Von D is the poster girl for the tattoo industry, but what she has at this point is pretty much still a cult following, albeit one that really loves her. It will not be possible to attach a value to her autographed material until she has been around longer. With her book tours and product promotional appearances, she is putting enough autographs in the marketplace to satisfy demand. As long as she is signing like crazy, prices are going be low, really too low to estimate.
If her popularity outlives her reality show, and she slows up her signing, prices will rise.