LOS d: Explain how the marginal cost of capital and the investment opportunity schedule are used to determine the optimal capital budget.

The “marginal” cost refers to the last dollar of financing acquired by the firm assuming funds are raised in the same proportion as the target capital structure. It is a percentage value based on both the returns required by the last bondholders and stockholders to provide capital to the firm. Regardless of whether the funding came from bondholders or stockholders, both debt and equity are needed to fund projects.

The marginal cost of capital represents the cost of raising an additional dollar of capital. The cost of retained earnings would only be appropriate if the company avoided creditor-supplied financing or the issuance of new common or preferred stock (and preferred stock financing). The after-tax cost of debt is never sufficient, because a business, regardless of their size, always has a residual owner, and hence a cost of equity.