Variable cost increases as production increases. The amount of variable resource employed, and the price of the resource, determines the amount of increase in the variable cost. An important distinction needs to be made between labor rates and labor costs. Labor rates and labor costs are not the same thing. A labor rate is total salary divided by the numbers of hours worked.
Labor costs take into account productivity.
However, costs may increase significantly because the new, lower-paid employee may be inexperienced, slow, and less capable. What is important is the employee's relative cost, and this is determined by the employee's productivity. Marginal cost is the single most important cost factor when determining how total cost changes as output changes. Marginal cost is the change in total cost resulting from a one-unit change in output. Numerically it is the change in total cost divided by the change in output.