I am not a designer, just a hack illustrator. I created an illustration that someone wants to put on their book cover. How do I even know how to price this thing? I got an email from the publisher asking if they can use it and how much would I charge. I have no idea what to do next.
Think some possiblities:
The book is a world hit, a million copies is sold in the first year and the foreign translations also have your image in the cover. You had sold unlimited exclusive rights for a flat $100,-
The book is a low cost paperback, no-name writer, only 2 thousands printed ever.
Probably the publisher is not eager to tell too much details (=what book, how many printed) but wants to get the unlimited exclusive rights to manage the image forever for a flat single payment, demands fast response or they take another from elsewhere.
- only limited time rights with a clear ending date, after that day the image is yours and only yours again, no longer than 3 years.
- no rights to resell or rehire the rights without your acceptance. Specify that you are the participant also in all possible reselling or rehiring cases.
- hire to them the right to print for example N copies for a single payment, then next payment for every starting N, no rights to use elsewhere than in the named book cover and in the materials that directly advertise that book.
I have no idea how many books you can realistically expect to be printed, but be sure that you get at least 3...5 x the sum that you consider to bee good payment for the used time. This is valid, if there's no hope for a hit. In all cases demand the 1,2 and 3 in written and signed form.
Consult a specialist who knows the local law. Do it!