Michigan Bank Drops Prime to 7%: National Bank of Royal Oak, in Royal Oak, Mich., lowered its prime lending rate to 7% from 7 1/2%. No other banks are known to have taken similar action. Bank President Edward B. Lefevre said: "Our cost of funds, based largely on the costs of short-term Treasury bill rates, have continued to decrease. The continuing decrease in interest rates for deposits makes it possible for us to lower the prime." The prime rate is the interest banks charge their most credit-worthy customers.