Mikael Karlsson , Dec 24, 2009; 04:42 p.m.
Why is exclusivity needed? When I license covers it is strictly one-time, non-exclusive license. If the publisher is willing to pay double the regular fees I might consider exclusivity for the topic of book they license the image for, but only for a very limited time like a year or two at the very most.
Also, is the writer self-publishing? If not you should be negotiating with the publisher. They know the ropes, what the "normal" fee-structure looks like, what type of license is needed etc.
As a very general guideline, I typically start my pricing for a 40,000 copies print run, one-time cover rights, North American and one language rights at roughly $1k and it goes up from there depending on a wide variety of factors and no e-rights at all are included in the base-fee. This would be for regional as well as national US book publishers.