Hmm.
I do have to point out that your increase in gold% in any city, be it from Trade Post, Bank, or any other source, doesn't cost you more due to any science buildings, at all, as long as you have a positive income in said city after all buildings you want are built.
It's simple math really.
Say you have a total of 40 gold base in from CityI. Counting with a really low number to show you.
On top of that you have some direct -gold buildings, 10 minus in gold total, mainly for defence purposes and -crime and +happy/health. That leaves you with 30 base gold (everything is always counted from base).
Say -15 from crime too for +15 base left.
Now you get some science buildings. Library and School of Scribes, for -8 gold, or +7 base gold.
With nothing increasing your gold% you are left with 7 gold per turn.
Build a bunch of +% buildings, there's loads of little additive ones as well as the big ones, for a total of +25%.
7*1.25 makes for 8.75 gold per turn. Your increase in % might have increased the "real" cost of the science buildings but you still get more gold because of those +% buildings.
Add a Bank on top of that and you have 7*1.55=10.85 gold income per turn. You never lose more money by building increased gold% buildings, regardless of their base costs, as long as your base gold remains in the positive.
I don't get why people keep stating this. It's just not true.
Also remember that I counted really low. Get your base up a bunch more; a couple Merchant Specialists, more pure +gold buildings, Civics that help out, and you'll simply gain more, never less, from +gold% buildings despite all those -gold buildings that "suddenly" cost slightly more.
Remember, always count from the Base.
Mouse-over your income in a city to easily see your total base from buildings (and specialists) before %'s are added. That base includes all + and - gold buildings that city has, counting crime as well as those are "buildings" too. A +30% from Bank is +30% on THAT base, ignoring all -gold buildings you might or might not have.
Cheers