The money income total/tooltip is powered by witchcraft........
Or that's what it looks like to me.
I have tried to ignore it each turn, only tweaking pop if it is showing a large negative income. I can't any more, I need some stuff explaining to me

An example from this last turn. Playing as Britonae, leading in legacy.
My money toolip was as follows:
Total 2296 +216
Reg com 8525
Cit tax 4596
Mil Upk -3376
Admin burden -9539
Ok, that looks good for next turn doesn't it, no need to worry.
I process the turn and hey presto......
Total 1292 -149
Reg Com 8174
Cit Tax 4617
Mil Upk 3376
Admin -9574
Why? Nothing happened. I am not at war so no modifiers to lower my tax/trade. I didn't take any options that cost me lots. Ok, regional commerce has gone down by approx 350, but that is the only thing that looks like it has changed.
How did I lose 1000 money?
I think the tooltip should reflect your expected income for the next turn. At present it is just guess work.