Hmm. I believe -2 is the hardest which uses a modifier of 1 x base = 5000 for a school, 7500 for college, etc. Prussia starts out with a college, so that would be consistent with 7500. Perhaps that's it
On the topic of bankruptcy...
It would seem that the base income for minor nations has been reduced by something like 40% since the 1.3 patch. This makes the impact of research cost for minors even more likely to put you in the red, particularly if you try to grab a region with an existing school. You still have to pay the research cost for one turn even if you raze the building immediately and covering that cost is now well outside base income on any difficulty. Also, if the building becomes occupied, you can't demolish it until you boot the occupying army. Playing as tiny Wurttemberg on VH has become even more dismal (which is fun)
I was wondering, however, if it might be possible to alter this scenario somewhat. Is it possible presently to read the value for turns remaining for the current research? If so, a 0 value (or whatever it is when nothing is selected, building damaged,etc) could be used as a condition where only a fractional upkeep cost is deducted. This would make it possible to suspend research funding when needed and also not get caught in implausible situations where you're paying for research that you aren't actually conducting. I like the planning you need to do to acquire and maintain schools, but have a hard time rationalizing the need to demolish the most expensive buildings just to stay under budget (or the inability to do so).
Anyway, just a thought. Enjoying the mod as always