On the other hand, if your Badboy rating isn't too bad and the other country's enimity is based on general dislike instead of specifics, your money goes a long way. I moved Denmark from being -175 to me (Russia) to +200 with only about 400 ducats spent over six years. Now they are allies and a fine counterweight to Sweden and the Tuetonic Knights.
As stated, religions can have a big effect. Protestant versus Catholic (any type), Moslem versus Christian (any type) seem to require major expenditure and a long term commitment to 'making nice' to have any effect at all. I've also had no luck at all in moving China's - 190 to -200 rating versus Russia regardless of what I spend on them. I conclude that the Chinese simply can't stand any European who settles within a province width of their borders, no matter who they are!