Maybe I'm remembering something else, maybe not. And the search function being what it is on these forums...
If you Mesmerism/Sorceress the Mercenary, do you have to pay 1G to take him as Follower? If Dungeon Keeper has Mercenary, you have to pay 2G (1G for DK, 1G for Mercenary)?
My brain keeps flicking the "yes" light.