How about
I = Country Income excluding the whole country bonus.
U = Reinforcements available.
10 * I + 100 * U
Montenegro have 5 income and 1 reinforcement:
5 * 10 + 1 * 100 = 50+100 = 150.
India has 107 reinforcements, 1372 income:
1372*10 + 107*100 = 13,720 + 10,700 = 24,220???
Same formula produces +100 difference when picking Ukraine, too. 3,400 instead of 3,300. However it works just nice for countries with <3 reinfs.
It is probably potential...