For underplayment of labor, you determine what a living wage would be in the area where the underpayment takes place, subtract the actual amount paid, and the difference is the "cost" of underpayment (which shows up only as a net positive in any actual balance sheets, but imposes net cost on society as a result of having more people who struggle to survive).

You could go further and quantify the effective costs of that struggle, but I don't think you need to. If something you buy contains 2 hours of labor that should have been paid $8/hr but was paid $4/hr, then there's a component to the "true cost" of at least $8.