I ran into a problem recently with money.
-Early on I was good, I was able to trade with most or all AI for full value of my goods.
-Early on I had little maintenance fees for units, terrain and buildings
-Early on I was only able to work a few tiles, so my income was low but overall I was doing well.
-Late game I was doing bad, most of trades with the AI didn't exist anymore or was a reduce value.
-Late game my maintenance per unit was high, the amount of buildings I had created a high cost and my terrain maintenance was high
-I had a market, bank, treasury and some policies which helped with money
-I was working a lot more tiles now
--- The problem was that most of those tiles didn't have the single gold I needed, it was all grasslands and only a small portion of my cities bordered "fresh water"
I simply was working enough "money tiles" late game for the amount of maintenance I had.
I needed to switch the 4F grass lands to 2F 2G and slow/stop growing my city.
This was a unique problem as I normally warmonger and puppet city states automate workers (who generally like to build trading posts I find, probably because the puppets are set to gold focus).
This time I was trying a 4-5 "mega" style in which most of my land didn't have tiles that was near a river or lake.