Well, not to be a pessimist, but using the artist's website is not the best gauge of value.
Current offerings:
Aspen #4 TEXT $5,000 on Benton's website. $750 on ebay with a $1500 buy it now.
Aspen Wall Poster #4 $5,000 on Benton's website, $850 ebay Buy it now after failing to sell recently for $850 and $450.
Recent closed auctions:
Hunter for Sheriff #5 Offered for a whopping $15,000 on Benton's website, Sold on ebay for $875 last week and another for $1075.
It seems that a good gauge of value is to take the price that they are offering it for on the Benton website and figure that you can buy one (or sell yours) on ebay for 10% of that. I know that using ebay is a shit way to value art, but I'm not sure who would buy a poster for 10x the price that they are being offered for from week to week.
Now, yours is certainly more owing to the signature and thumbprint, but you should use the realistic base price and then add to it for the rarity instead of starting at the inflated price and working up from there.
Bill