The cost of upgrading a unit can either be the DIFFERENCE in the cost between the two units, or the FULL price of the unit you are upgrading to. Is this correct?
A couple of examples (there may be more, but I was starting to get a headache
Upgrading Wehrmacht Inf to Grenadier - Cost = Grenadier cost (100) minus Inf cost (90) =10 prestige. However upgrading to any other type of infantry unit costs the full price of the new unit.
Upgrading Pz36t to PzIIIf - Cost = PzIIIf cost (280) minus Pz38t cost (238) =42 Prestige. Any other upgrade costs the full price of the new tank.





