Perhaps $50/hour for the creation time for the image. And that is still cheap... Is the Image going to be used as promo, posters, book promo, online image? Should there then be a license fee for the use of the image? How many copies of the book will be printed, and how many different editions/languages? Keep the Copyright and lisence it to the publisher for a term of use or a set number of copies/editions? You have a nice image, ask for a specific size font for the credit of the book cover art and set the value of your art using the terms above. Have an international money order in your hands before you sign a release to allow the use of the image.