It follows the murphy system. The software reads your mind, and improves it the least helpful way.
In all seriousness, every time I think I know what it's doing, something changes. The cost typically is based on the use of the NPC for bribes, and the existing level of the BOD's 3 properties. Larges cost more than smalls, exceptionals more than normal. I wish the bribe values reflected which property was going to be raised, but I don't think that gets determined until after you drop the BOD on them.
I think it gives a color change (if possible) more weight, then normal/exceptional, then the count.
Personally, I'd prefer the order reversed, for some skills.