I've got no experience selling dragons with art, BUT as a fellow artist: if you put a lot of work into it, hell yes raise that price. You deserve to be compensated for your efforts! Plus, it's not like you agreed to sell someone the dragon ahead of time for x amount -> THEN decided to raise the amount because you wanted to add art, so there should be no guilt in raising your cost to account for time spent.

And hey, if worse comes to worse and you don't get any bites, you can always lower the price- but you won't have the opportunity to raise it. I'd say go for the higher cost!