If you have N C17 then they cost £X whether they fly or not.
If you need the capacity for less than 100% of the time then you are wasting money. What happens then is the aircraft are taken in to Reserve, ie spares, manpower and fuel are not provisioned.
If you reduce your holding then the cost of ownership will reduce. To then buy in occasional charter to make up for your smaller GOGO fleet then it has the potential to cost less.
Chartering for Herrick is a different ball game as it is effectively a whole-fleet charter rather than an occasional fleet make-up.
On the leasing of C17s, I believe originally it was for a specific period of time and a specific rate of usage, bit like your contract hire car. As Herrick ramped up so did the hours and the overrun leasing charges would have been greater than the option to buy.