Experts agree that 50% of manhole leakage occurs in the chimney section. It is an accepted fact that manholes can leak up to 10gpm for an annual per manhole cost of $8000 to $9000. 50% of these figures, or $4000-$4500, will represent the chimney section annual leakage cost. For your system, you pick your per manhole chimney section annual leakage cost, then pick the number of manhole repairs, and see your annual savings.