I was approached by a book publisher I know today and he asked me if I would be interested in the selling the rights to one of my images for a book cover. I told him I would be interested. He informed me that he sent the image to the author and strongly suggested using it for the cover. He then added that he doesn't know if the author will or not because of costs. That's where we left it.
I have to be honest here, I have no idea what to charge for something like this. From what I have read so far, I know I need to retain the copyright and request that I receive credit for the image. I assume this will be a single edition. Therefore, lets say this is a single edition of a book with my image on the cover.
Please share with me what YOU would charge and why?
Also, I understand it is important to define a period of time of use. However, with this being a book that could sell for years to come, should I limit the use to X years or just go with a flat rate fee?
If we do agree to say a 2 year agreement, that means I can not sell that image to anyone else for two years, right?

