As for the savings. Just a phone call to the image owner or agent should quickly let you know the viability and estimated cost of the revised image that you have in mind. If you are dealing with a stock house, both their original commission and the illustrator's time to make the change will factor into the final pricing. Dealing directly with the artist should save you 20%-40% of the cost and time necessary to do the original image. For best results when negotiating such an inherently unique situation, we all do well to keep the basics of both the buyer's and the seller's positions in mind. Of course, without the buyer there probably won't be a sale in the immediate future. On average, I've found that one of my stock illustrations sells only 2 or 3 times a year. On the other hand, the buyer stands to gain the inspiration, a finished illustration and some custom work to make the concept work in a new way.
Creative direction, art direction, design and illustration*
This image can be ordered from the Attiliis collection of illustrations at Stock Illustration Source