Add possibility for a unit to have a reduction of total cost of attached units

[Vincent Lhote]

I would like to be able to have the ability for a unit to reduce the total costs of attached units, without putting the total below 0.

For example, unit A has a total cost reduction of 5, and its cost is 0 points. It can have any number of unit B, costing 6 points, and unit C, costing 4 points, attached.
If no unit are attached to A, then the total cost is 0 point.
If one unit B is attached to A, then the total cost is 1 point (0+min(6−5,0)).
If one unit C is attached to A, then the total cost is 0 point (0+min(4−5,0)).
If two units B and one unit C are attached to A, then the total cost is 11 points (0+min(2×6+4−5,0)).