Final answer:
The fundamental factor affecting the cost of money in the scenario described is risk. By setting the condition that the expected return on the investment should be at least four times the return on Treasury bonds, the investor is demanding a higher return to compensate for the higher risk associated with investing in a business.
Step-by-step explanation:
The fundamental factor affecting the cost of money in the scenario described is risk. While inflation and time preferences for consumption can also influence the cost of money, in this case, the decision to invest in the friend's business instead of Treasury bonds is based on the expectation of a higher return on investment.
By setting the condition that the expected return on the investment should be at least four times the return on Treasury bonds, the investor is demanding a higher return to compensate for the higher risk associated with investing in a business.