Boeing 707 burns about 6500 KG
of fuel for first one hour of flight if your departing with maximum T.O weight & as you know fuel consumption decreases as weight decrease, also I would like to note that fuel consumption of the 707 is equal to the airbus 300.
I would like also to note that the 707 can take up to 40 tons of payload that is equal to the Airbus 300.
About spare parts they are the cheapest in the market & so simple to find.
I also want to mention that the Boeing 707 is a money making plane till now for cargo airlines.
I'm sure Beta Air is pleased with their Boeing 707 operating costs .