@Rich I certainly agree that the more expensive the car, the more chance that it will be cheaper to run on Motability. Equally, cheaper cars can be significantly cheaper to run privately.
On another thread I have mentioned that I am buying a new Hyundai i20 for one of my daughters to replace one that I bought her new 6 years ago. Taking into account depreciation, servicing, maintenance costs (tyres etc), insurance, road tax etc, the car has cost in total £1,300 a year to run. Had she been a Motability customer, even if the car had been zero AP, it would have cost her £3,000 a year.