I had that before, on .291.  Captured some enemy cities and my treasury went negative almost immediately.  I reported it in this thread.

Last build this happened to me everytime I had more than 6 cities. Your 'city upkeep' skyrockets whenever you have more than 6 cities it seems. city upkeep goes from something like 27 at 5, 97 at 6, and several hundred at 7 though i haven't checked how much this has changed with the new build.

Similar thing happened to me -- I noticed it immediately after taking over a few of my opponent's cities (2-3ish, with a total of about 6 cities).  A few turns later my treasury was at -400,000

Ok, I got that one !

The first city has an upkeep of 1, the second an upkeep of 2, the third an upkeep of 3.

So the total upkeep should be : 1+2+3+4+...

But !

1 city -> 1 upkeep

2 cities --> 2 upkeep

3 cities --> 6 upkeep

4 cities --> 24 upkeep

5 cities --> 120 upkeep.

Do you see what happenned ? Someone need to sleep, cause "+" and "*" aren't the same operation

Instead of adding the upkeeps, they multiply them !

6 = 1*2*3

24 = 1*2*3*4

120 = 1*2*3*4*5

6 cities ? upkeep of 720 ! More than prohibitive

EDIT: in fact it's 0 < 1 < 2 < 6 < 24 < 120 < etc. The first city is for free.

Interesting, though I’m pretty sure it’s intentional. Crude but intentional. I’m sure its a way to limit city spam and eventually promote the creation of vassal states if you want to expand your power. They might find something more elegant solution to city spam as the beta goes.

Maybe the sovereign could lower the maintenance cost of city the longer he/she stay there. Each turn it goes down until it all reaches a baseline. This represents the sovereign getting hands on with the city, reduces corruption and making sure everything is going as planned. Then the sovereign goes to the next town to do the same. The prior city's maintenance starts going up but at a different rate. This rate can be lower or higher depending on sovereign/faction/race attributes.

It basically gives you something else to do with your sovereign while not at war. Seems my sovereign is just picking his/her nose when I focusing on economy and infrastructure.

seems like they forgot to fix this while they were busy making the game more stable.

why would you need more then 6 citys the ai is stupid as it is... with each build the game gets better lets hope they don't forget something major.