I've looked but haven't been able to find anything in the XML that controls the cost of upgrading a unit (say, from warrior to axeman). Ideally, I'd like to have the ability to either change it globally, or with specific buildings (i.e, city with a barracks makes an upgrade 20% cheaper; West Point adds another 20% discount ...). Anyone know how to do this, or is it a Python-only thing?

thanks in advance ...

It's in GlobalDefines.xml. Lines 881-888.