Help with my Roman Empire custom map

After being inspired by Greek World, I decided to make Roman World. The only problem is that I have no clue how to make any suitable income due to no source of GDP for the cities. No, I cannot do today's become because many of the cites were much less important or don't exist anymore. So I did it based on population. I took everything but the last two numbers of the population and divided it either by 8 for a port or 9 for an inland city.

What I tried to do after two game with shit income is double it. I didn't even need to play to figure out that this still won't work. The problem is that there are some major cities such as Alexandria, Rome, Jerusalem and Antioch which have large incomes and would be of disproportionate income if I doubled the, certainly going over the max income which Rome does without doubling.

The only other option is to make units worth less which I don't want to do because it would cause confusion. For example, you see a ground unit and don't know what it is, but when you see 120 cost you know it is equivalent to a tank. If I lowered it this might cause confusion.

Any solutions?

Dont try to make it realistic at all, try to make it fair.