Many people don’t have a clear idea of what their debt really costs them, whether it’s a credit card balance or a student loan. Financial writer Gail Vaz-Oxlade points out in “How Much is Your Debt Costing You?” that if someone buys a $2,000 TV on credit, with an 18% interest rate, the minimum monthly payment would be about $40. Of that payment, only about $10 would go to the principle and rest would cover interest. So if you just made minimum...