Final answer:
The cost of capital estimate for evaluating Ameritrade’s upcoming investments is 4%.
Step-by-step explanation:
The cost of capital estimate that could be used in evaluating Ameritrade’s upcoming investments can be determined by calculating the firm's effective rate of return. In the given scenario, the interest rate is 9% and the cost of financial capital. The firm can capture a 5% return to society. To find the effective rate of return, subtract the return to society from the interest rate: 9% - 5% = 4%. Therefore, the cost of capital estimate is 4%.