It's interesting; I get the same reaction from the pope if I promise a tribute of 200 fl for 10 turns as I do if I give him 1000 fl in a lump sum (haven't tried 2000). I wonder if he adds it up every time making it look like you're giving him much more than you really are?

The danger of excommunication is that everyone and his brother can attack you (and will). Your allies will turn, and crusades can be called against you. Unless you like living dangerously, excommunication is to be avoided.

What is effective is provoking the other side so THEY get excommunicated. Move a small stack of expendable troops next to one of their larger stacks but don't attack. The AI can't resist. Keep doing this and soon your foe will be on the outs with the pope and you might even get to call a crusade against him. Heh.