Change increased coring cost depending on core numbers.

[Belidonk]

Increased coring costs represent the willingness of a province to subjugate to the will of a new overlord. But IMO if a province was part of many countries there should be a more "relaxed" attitude to be ruled by another ruler. In some cases some provinces have over a half a dozen cores. Such a province should be more willing to be part of another country then a province that has only one core and has never experienced other rulers.

 

[tobias.mb]

You could just as well argue, that people constantly under foreign control / oppression, will eventually get tired of it and want to be independant / controlled by their "natural" (primary culture?) nation. Thus making core creation in such provinces more expensive.
And it would devalue a rather weak modifier even further.
Maybe a "recently conquered" modifier that reduces warscore cost would make sense (having just conquered it, the nation should not have a firm grip on the province yet). But that is already kind of done by autonomy reducing warscore cost...