To simplify the PIP Mobility enhanced rate and the older DLA Higher rate mobility allowance are both £62.25 per week
£62.25 Per week (£269.75 per month) this is the maximum that the DWP would send direct to Motability. Some vehicles on the scheme are less and the remaining difference would be paid directly to your bank along with any care allowance you have been awarded.
If you choose a more expensive vehicle then you would pay an AP (Advanced Payment) to the Motability car dealership. The AP is set by Motability and the car manufacturer (although it is possible to get a discount on the AP by negotiating with the dealers or some dealers do advertise special offers – see other threads on here for current dealer offers