There is no maths to back this up, the cheaper cars are funding the more expensive. Simples.
If we all got a Tiguan the Scheme would be out of business in a year, you have to factor in the huge costs of Motability Operations, rent, wages, insurances etc not just the cost price v residual price.
The fact that people choose fiestas and polos make it possible for the Beemers and Tiguans to be on the Scheme.