Exploit: unit size upgrade too cheap
An exploit in unit upgrade costs allows one to save a lot of resources. Create units always in the smallest possible size, then upgrade to larger size instead of building directly the larger sized unit. The upgrade costs always 30gp per unit size step, so for some amount of gold you can save up to half of the unit's real material cost. I think the unit size upgrade should cost 30gp + full material cost for the unit's equipment.