Cost of upgrade

[Ljube]

I'm doing lots of upgrades and know it's 60 to upgrade a warrior to sword, 30 more to upgrade to medieval infantry and 120 to upgrade from horse to knight.

Can someone tell me how these costs are calculated so I could apply the same equation to other upgrades? Usually it means very little, but sometimes calculations of this kind are crucial.

Thanks!

[Kloreep]

3 times the difference between the shield costs

So Knight-Chariot=50*3=150, Pike-Spear=10*3=30, etc.