 When interest is charged on the premiums, the cost of a single premium policy increases the cost geometrically.The above secured loan of £25,000 over a 25-year term at 4.5% interest costs the customer an additional £20,221.74 for PPI.Moneymadeclear calculates the repayment for that loan to be £138.96 a month whereas a stand-alone payment protection policy for say a 30-year-old borrowing the same amount covering the same term would cost the customer £1992 in total, almost one-tenth of the cost of the single premium policy.