The Wall Street Journal prime rate of interest is a published indicator of the rates that banks charge for loans to their best customers. This would be a good rate to use in a NPV model because: For a major corporation, it is probably higher than the actual rate at which the company can borrow, and the resulting NPV is conservative (under-estimated). For a small farm or small business project, the prime rate represents the cost of debt. The investor's true opportunity cost of capital is equal to the market interest rate. It is a reputable source of market interest rates and reflects the cost of debt