The savings could translate to enormous savings on heating bills.
Check the doors to see if there are leaks. They are often not closed properly and warm air escapes. You will usually feel hot air leaving by placing your finger on the corner. The solution is simple. There is only one requirement for door seals. They will be attached right at the edge of the door. You could also use spray foam for this instead. Spray foam is an alternative that keeps your warmth within and will keep your wallet full.