Scenario:

You are the director of operations for your company, and your vice president wants to expand production by adding new and more expensive fabrication machines. You are directed to build a business case for implementing this program of capacity expansion. Assume the company's weighted average cost of capital is 13%, the after-tax cost of debt is 7%, preferred stock is 10.5%, and common equity is 15%. As you work with your staff on the first cut of the business case, you surmise that this is a fairly risky project due to a recent slowing in product sales. As a matter of fact, when using the 13% weighted average cost of capital, you discover that the project is estimated to return about 10%, which is quite a bit less than the company's weighted average cost of capital. An enterprising young analyst in your department, Harriet, suggests that the project be financed from retained earnings (50%) and bonds (50%). She reasons that using retained earnings does not cost the firm anything, since it is cash you already have in the bank and the after-tax cost of debt is only 7%. That would lower your weighted average cost of capital to 3.5% and make your 10% projected return look great.

Question:

Is Harriet's suggestion of using the cost of debt a good or bad idea? Why?