Publication rights?

[drakhen.livejournal.com]

Hi all. This isn't really a 'beware' message but maybe an 'aware' one? I didn't know where else to ask.

I've been approached by an RPG company (they look kosher) asking how much I would charge for the publication rights to one of my pictures. They stress they don't want full copyright.
*name deleted* is potentially interested in acquiring publication rights to use the attached image and possibly commisioning several similar pieces. Could you please send me your requested rates for this (publication rights only, not full copyright).
(it was this image http://www.deviantart.com/deviation/9317804/)

I have precisely no idea what the going rate for this would be. If any of you have any advice or can point me in the right direction for some then it'd be much appreciated!