I solved the EXTREMELY high corporation bug.
Basically, there is a new tag called <iRealCorporationMaintenanceModifier>, which, if it is defined, seems to always reduce the corporation maintanance(!). 13 civics have it:
Subsidised, Socialized, COrporate, Private, Environmentalism, Regulated, Corporatist, Planned, Mercantilism, Guilds, Coinage, Aristocracy, Proleterant
I was running Republic/Bureaucracy/Liberal/Free Market/Volunteer Army/Free Market/Paradise.
Basically, <iRealCorporationMaintenanceModifier> is the 'tax levy on corporations' modifier. HOWEVER, it is currently bugged, as the person doing the XML wasn't talking to the person doing the python code. By the looks of things, according to the XML, a value above -100 is equal to a tax subsidy on a corporation (the percentage is equal to the difference), and a value below -100 is a tax levy. End result: Green has -150 and corporatist has -75 (with a 50% levy and a 25% subsidy respectively)
However, instead of the default value being -100, it is 0, which means a 100% subsidy!!
Switching to corporatist reduced the corporate maintanance from -10.21 and -18.64 to -2.55 and -4.65. Ironically, these are 75% reductions in maintanance costs (despite the Civilopedia saying it should be a 25% INCREASE)
So, in summary, <iRealCorporationMaintenanceModifier> is completely bugged. Ultimately, my 'solution' is pretty harsh XML fiddling. Firstly, EVERY value of iRealCorporationMaintenanceModifier needs to be increased by 100 (which would make the civilopedia incorrectly say that corporatist gives a 125% subsidy, instead of 25%), and maintanance costs of corporations need to be decreased by a HUGE amount (I am thinking about having a 75% reduction in maintanance costs, although 50% might be reasonable)