I do not know about wrong though a few of them are a patching over instead of addressing issues.
First off I figured City Maintenance costs in C2C were way low as they were not really buffed up between versions even if new +gold buildings were added so I tweaked them a little, then a little more, and when a little did not make much difference I set to increasing them a lot, then a lot more, until I got to the numbers I posted earlier in this thread, and tweaked more with more testing:
Due to this also setting initial City Maintenance of several gold at year 50'000 BC and the very first city I needed to do something about that to reduce the Maintenance in the start, during PreHistory. I also believe money should not really matter in PreHistory but for game-play sakes it is needed.
So I invented the PreTribeMaint (bad name, I knew it then too, just supposed to be a place-holder anyway) that gives -90% Maintenance to the city, and limited it to maximum of 2 so a third city would cost full maintenance. It auto-builds in Anarchism governments and is removed when switching to Chiefdom.
With increased City Maintenances all the large changes in Civics made a drastic difference so I went through and tweaked them a bit, lowered most values and removed a few from civics that maybe should not have a part in City Maintenance at all.
I think most Rule Civics should have their changes to Number of Cities halved still though after having done a few more games with the changes: 25 percentages to 15's and 20 percentages to 10's, 15's either to 10's or 5's, and 10's to 5's. Distance looks good or could even be increased a bit as that has a much lesser impact on total costs.
The same really goes for other civic areas too but to a lesser extent.
I also reduced the +City Maintenances the City Halls buildings to +5, +10, +25 instead of +10, +25, +50, and set Chief's Hut to give -10% too for a slightly better progression from Anarchism to Chiefdom.
That last is sorely needed as otherwise a really huge hit in economy suddenly comes when switching to Chiefdom. Even so it is a big hit (and the changes to Shoplifting (crime) did not alleviate that) and a better progression would be very good to have.
I also increased initial starting free units from 4 to 10 (which I think you reverted to 4 again) so that the by now very much reduced income in the start (from all the -xx% gold, not from my changes) would not hinder a nation to save some animals or sport a small army if need be. Some and small being the key words.
Secondly while asking around in forums and fixing that I found out that there is a mechanics to increase the maintenance due to the number of cities a player has. Looking at it I rather quickly realised that that mechanics had not been touched despite C2C both being very heavy in the adding all kinds of things, science, culture, hammers, and of course, gold in buildings, and also being very friendly to having big empires (even if using revolution).
A few tests and checks later I saw that after reaching this cut-off from additional cities no longer increasing maintenances due to number of cities there no longer was a loss in placing a new city, not even if placing 20 new cities, as the "more is more" free buildings auto-built by a new city was higher than the maintenance cost of that city.
I did a "show and tell" test that I posted to show that effect in Bugs and Crashes here:
http://forums.civfanatics.com/showthread.php?t=530432
Basically once having reached that limit there is no negative effects from expanding to fill the world up at all, especially if building gold buildings in all new cities.
Possibly having a lot of the buildings that cost a lot in maintenance would increase costs so that this kind of expansion would not be as easy (but still possible) but catering to all play-styles should not mean a no negative building/hyper-expansion strategy wins over any other. In my opinion, and I like expanding a lot myself (which is how I could find this).
That figured out I changed them slightly. Then more. Then a lot more. The Handicap file in SO's Nightmare option shows the final derivation but those have not been fully tested as I only run in Deity (now gone down 2 steps in a test game though) so I have not yet reached the new cap. I suspect though that they could be tweaked to be maximum 30 in Deity and 10 in Settler, or thereabouts, and somewhat progressively between the two, though in most games the upper cap of 30 would seldom be reached in Deity either.
I know how a lot of people say that settling a new city should not increase the cost in the old cities, which in a way is true. What does increase for every city though is the overall costs; political, administrative, state, bureaucratic, tax-collecting, basically most of the civics increased costs. Since that has no own place I suppose the increases for all cities reflect that somehow. And no, the Civic Costs are not increasing by more cities needing overseeing, it is a fixed sum per city depending on it's size, not on how many one has.
If the added costs can be set there though instead of in each city on it's own that might work too, but figuring out how to change it by amount of cities might be hard to do and balance, and as I am no modder I can not even begin to know how that would be done.
Oh, and if using an increased City Maintenance cost the arbitrary City Limits can be dispensed with, I think.
I probably missed a bunch of things but that is my overall thinking, albeit shortened.
Ask if you have any questions.
Cheers
for once when expanding too fast. So I was forced to build more 
buildings that I normally do. Or I should say build them earlier than I normally do.
in maintenance change (16 cities, NOT Deity) so I would say 15% and 10% are still having a big impact with the higher MaxNumCity.
