"Cost multiplier" in the free-to-download game C-evo is a term basic to the construction cost of any unit the player may design. It is the highest number specified for any ordinary advance you have discovered, increased by the amount specified for any future technology advance you have discovered. It is multiplied by the number of modules in the proposed unit, to give the total construction cost of the unit (excluding other enhancements that are not modules but have their own cost requirements).