Heres how bribing should work for cities. It should be based on distance to capital and the cultural difference. This way Barbarians would not have a chance to bribe Italian cities or Greek cities in the heart of the empire.

Heck the distance to capital shoud be the same for troops. Imagine a legion getting bribed right outside of Rome.

The only new decent bribing tactic I've seen by the A.I. is now they will try to bribe islands instead of attacking them.