[BUG][.915] Negative Gold Cost for Rush Production

From what I understand there is a three turn minimum on build/train times. 

You can "rush" production by paying for the remaining hammers(Production unit) with Gildar(gold).

The cost, in Gildars,  is equal to the remaining hammers needed to complete construction.

When I build something that is restricted by the turn limit, the rush cost becomes negative due to the "over produciton".

For Example, I was rushing "slaves" every turn at a 50-100 gilder profit per unit. 

I think the negative impact needs to be removed, or maybe add a base cost to rush something restricted by the 3 turn limit.