I don't particularly care if the cost goes down but how about this for a better effect:
1) Never Surrender: gives the province an unrest modifier equal to ceiling(cost modifier / 10) (i.e. 50% hostile coring cost gives 5 unrest). This modifier is permanent until the hostile core is revoked
2) Strength of the Underclass: gives the province a rebel strength modifier equal to the cost + 100% (i.e. 50% hostile coring cost means that when rebels spawn, 50% more rebels appear than normal (20 rebels normal = 30 rebels with the modifier)). This modifier affects all kinds of rebels spawned from the province
3) Whispers inflame rebellion: Neighboring hostile cores give an unrest modifier equal to the number of neighboring hostile cores from the same country, this modifier doubles after the printing press is invented. So if you conquer three provinces in a line from Bohemia, the two endpoints would get +1(+2 with print press) unrest; while the center point would get +2 (+4 with printing press) unrest.
This would make revoking cores a very important war goal.
