Liguria, by virtue of the trade node changes, is now perhaps the most valuable province in the whole game. Coring that for roughly half of what you pay for an idea or one tech level is not much. Unless you are already hugely powerful, owning a cored Liguria is without much of a doubt more valuable than one administrative idea or one tech level.
Coring costs for Muscovy's traditional targets for expansion are irrelevant to this discussion, because they are not nearly on the level of a North Italian province.
I know that history is not necessarily a compelling argument for game design decisions, however in my humble opinion, it is not necessarily bad game design if the game actually gives a Spanish, French or Austrian player a reason to think twice about whether to annex Genoa or to keep them as a vassal, as Spain pretty much did historically.
TLDR: For the love of god, please DON'T lower the coring costs. They seem fine in 1.12.