I believe an additional reduction in espionages costs or upkeep rates of spies might make sense
I think that instead of reducing unit costs, it would be better to decrease consumption rates of units and also increasing the project cost. At the moment it's quite useless for raiders who are mostly raiding with soldiers only. With the increasing amounts of projects, people would be reluctant to buy military projects over economic projects since they are not providing significant advantages during wars. The use of this project in particular is quite circumstantial, even during wars which also limits it's usability. @Themoniaalready stated those reasons do I won't repeat them.