I think diplomats are rather expensive in upkeep (like all agents). So try to have as few of them as possible.
Also, if you chose the right settings, it's very possible to have black numbers right from the start of the campaign, without disbanding any units.
See, I'm making 624 denarii:
What I did is set taxes in:
- Bononia to High
- Mediolanum to Very High
- The two mini-region to Very High
The population growth in my cities after applying those taxes is:
- Bononia 0.5%
- Mediolanum 0.5%
- Mini-regions -1%
The mini-regions don't have to grow. They are designed not to do so anyway: they can't recruit any units, they can only build a trader and some roads, they will never be able to upgrade to the next settlement level.
You can make more profit (if you want to maintain a big army for example) by getting some trade rights. The Barbaroi are a good candidate (if you don't have trade rights with them from the start though, not sure anymore)
What I did here with the Cisalpine Gauls is basically what you have to do for most factions and you can be sure of a healthy economy.
I hope that helps
