The MOD cannot afford to buy new aircraft outright which is why they are going to lease them.
It is no different to an individual leasing a car. It is going to cost them more in the long run. The leasing company has to buy the car, recover their costs and make a profit (otherwise why bother?). If you include routine servicing with the deal it will cost extra.
Aeroplanes cost more to buy and run, thats why the figures are so high.