Of course military buildings need gold maintenance ... (if we are going with non-game examples, then to pay the wages of your Combat Teachers, Drill Instructors, Field Trainers)
It's perfectly sensible for % increasing gold buildings to also have a gold maintenance cost ... its less efficient to sponsor a "state-owned" marketplace in a small city.
Lets consider a training complex (for military) ... its going through practice/apprentice weapons and dummies every day. That could be a minor materials upkeep. Doesn't have to be, as you could just add to the gold maintenance their budget for "buying" such things.
so for training complex ... either 1 gold, 1 materials upkeep, or 2 gold upkeep (as an abstract and basic example).
A market could cost 1 gold for maintenance ... and increase Gold production by as much as 50 %
A bank could then also cost 2 gold maintenance, and increase Gold by 50%
.. in both cases, the gold is increased *before* the maintenance, so a city would need to make 4 gold or more for a market to become necessary.
A city would need to make at least 6 gold more for a Bank to become necessary.
however, assuming these bonuses don't add but multiply ... your gold is increased by 125% if you have both ... so assuming you were making only 4 gold, with both you would be making 9 gold ... and paying 3 in maintenance so its 6 gold .
However, as far as *fast turn around* on your investments ... you would rather like a city to be making say 10 gold, so that you would be making 22 gold, or a 19 gold profit, or a (9 gold) bonus. As opposed to 6 gold profit, or a (2 gold) bonus.
and of course, added to the % increases on city size, it'd only get better. (alternatively you could have markets use materials maintenance [leathers, furs, tools], and banks to use a gold maintenance)
Probably the construction of such gold-increasing buildings to be tied more to usage of materials. (cost more materials than gold, if costing gold at all)
And as to "Hidden" costs ... these costs wouldn't be Hidden, I'd say everywhere (in the tool-tips, in the building icon, in the Hiergammemnon) that a Market would "cost 1 gold per turn globally, and boost local gold by 50%" ... by tying the gold maintenance to the global treasury as opposed to local profits, you avoid the possible mistake of calling the penalty before the bonus.
But yea, the building itself would say (-1 gold, +50% gold) ... so its not a hidden penalty.