Go to General Properties in the Edit Rules and change the 'upgrade cost' number. The number indicated here is the multiple used to determine the total upgrade cost for a unit. For example, if the base unit costs 20 shields and its upgrade costs 40 shields, the difference is 20 shields. If the 'upgrade cost' in the editor is 3, then, the exact cost for the upgrade would be 60 shields (20 x 3 = 60).
This means you can set the upgrade to 0, where i think upgrades are free, or set it to 1, where you would only have to pay the difference between the two.