To change the recruitment cost of a unit open its unit blueprint like for example the BP_LumberjackUnit and search for Resource Cost like the image below.

The Resource Cost setting controls the required resources to recruit this unit.

You can edit the existing resources amounts or add/remove a resources to the Resource Cost setting. To add new options click the + button on the Resource Cost section, to remove resources click the down arraw and select delete.

Once you’ve add a new entry set the drop down menu to the resource you want and set the value required.