I once managed to persuade the Numidians to give me Libya in exhcange for a few bits of gold...

But like everything diplomacy only works when you offer the AI something huge, for it to even consider it. often it requires massive amounts of cash to bribe them off or something.

Really diplomacy isn't very good, all alliances get broken by the AI without consideration when it judges your doing too well, so all alliances and agreements are pointless. Also when coupled with the protectorate bug where the protectorate gets cash from you, there isn't much of an incentive to do it.

Still every once in a while you can get that peace treaty from the enemy, and demand a massive amount of cash tribute...