A generalized expression of the net advantage of leasing (NAL) is used to assess the implications of discounting incremental cash flows at the firm's before-tax cost of debt and the firm's after-tax cost of debt, respectively. If no personal tax biases are assumed, then the before-tax cost of debt should be used to compute NAL. If the before-tax cost of debt is the correct discount rate, then any change in the firm's borrowing level brought about by the decision to lease rather than purchase will alter the computed NAL by the amount of the present value of the tax savings on interest payments. Thus using the before-tax cost of debt is consistent with basic MM valuation theory. Using the after-tax cost of debt, in contrast, implies that any associated change in the firm's borrowing level is irrelevant for purposes of computing NAL. Sufficient conditions are specified for the after-tax cost of the debt to be the correct discount rate for lease versus purchase analysis. Finally, lease analysis in a MM world is compared to lease analysis in a Miller tax world. For the special case of a 100% leverage ratio, the specification of NAL is the same in both worlds. Use of the after-tax cost of debt is correct in a Miller world and is a good approximation in an MM world provided the cash flows are predominantly debt financed.
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