Usage Question on Book Covers

Hi folks,

I am currently negotiating for a picture of mine to be used as a book cover here in the US, and the usage is only for the first hard cover domestic run. Any interested international publishers have to negotiate separately with me.

My question is, in anyone's experience, what is your sense of what international usage rates are compared to domestic ones for book covers? More or less? Anything surprising? In this case, England may be asking, and I would like to go into a negotiation with some sense of parameters in order to avoid either over- or underpricing, the latter being my specialty.

Any advice would be great. Thanks!

Claudio