But the cost can be substantial. Most borrowers can't repay the loan on time, so they take out a second loan, which carries its own fees. And then there are the penalties for late payment or the risk of having one's car repossessed. In all, the cost of borrowing each $100 may be $10 to $30 every two weeks. Calculated as an interest rate, a mere $15 fee per $100 amounts to 391 percent over a year — at a time when even credit card interest rates hover at an average of 15 percent.