[Alneyan]

The Partnership income will also play a part (why did I forget this point?). Assuming there are four players in a team, who have roughly the same income, each player would actually have 40% of the income of the entire team. (If Trade Savvy is left on average)

A Partnership at 20% with a teammate bringing in 25% of the income of the team means +5% more of this income for you. With three partners, it means +15%, and +20% with four partners, thus doubling your income. So once the game reaches turn 20 and beyond (depending on when you can sign the treaties), your income will be much higher. (How much higher depends on the income of your allies)