From an article by "Detroit Free Press":
It costs Northwest 7.2 cents in operating cost to fly 1 mile per passenger on a 100-seat DC9-30, according to Eclat Consulting, an airline consultancy. That compares to the average of 6.5 cents U.S. carriers spend on flying 108-seat Boeing 737-500s, according to Eclat Consulting.
That should give you a general idea of the DC9's average operating costs. Also check out
this useful, if dated, operating cost reference...
Cheers,
QantasA332