
Milking the Senate and the People of Rome
Instead of sending a diplomat on a world tour to secure trade rights with piss-poor distant factions, why not leave him in Italy and task him with extorting money from your fellow Romans?
Start the game by offering to attack as many factions as possible (eight is the maximum the UI will let you haggle for). Neither diplomatic stance nor relations seem to matter.
The most I've managed to get this way is 1k for 9 turns from the Senate and 1k for 7 turns from my allies. In some versions of the game, once you follow through on your promise, you can immediately ask for a onetime payment in exchange for attacking the same target faction again.
And since the AI receives large cheat donations on higher difficulties, your allies will always have some spare money until you no longer need their help.