Kings Corp has determined that its before-tax cost of debt is 9.0%. Its cost of preferred stock is 12.0%. Its cost of internal equity is 15.0%, and its cost of external equity is 17.0%. Currently, the firm's capital structure has $600 million of debt, $50 million of preferred stock, and $350 million of common equity. The firm's marginal tax rate is 40%. The firm is currently making projections for next period. Its managers have determined that the firm should have $100 million available from retained earnings for investment purposes next period. What is the firm's marginal cost of capital (WACC) at a total investment level of $250 million?