These ones are purely for my own amusement so I don't do them to commission or sell them. I guess if a publisher asked for something similar, they would pay £800/1500, which the going rate for a book cover. It would be less if it was to appear inside a book. They take about 3 days on average.
Many thanks for the Daily Deviation honour, it's much appreciated. Thanks for all the comments; although I can't reply to all individually, it's great to get everyone's input. Thanks also to Warlock, for being an awesome model.