Because of the extremely high interest rates , credit cards are one of the most expensive ways to borrow money. Even so, at least one-third of credit card users carry a balance from one month to the next, according to the Federal Reserve Bank of Boston. However, a new paper published by the Boston Fed found that when credit card interest rates change, cardholders adjust their spending accordingly. On average, a 1 percentage point increase in the annual percentage rate, or APR, on a credit card