The wiki agrees with you (
https://wiki.project1999.com/Greater_Conjuration:_Earth), which begs the question: does Nilbog know something the wiki doesn't (eg. the spell used to cost more, then in Luclin or later its cost was reduced, and the wiki reflects the
unclassic cost) ... or is it a bug?
Given that Nilbog recently did an overhaul of mage pets, it
seems like it's probably the former, but everyone makes mistakes, so a bug is still a possibility.