The tension is that the PRM charge is typically negotiated with the airlines.
The costs of the service are negotiated between the airport and the PRM provider. The airport then divides the total cost by the number of passengers it expects to come through its doors and levies a per passenger fee. It is normally ringfenced from the normal commercial negotiations between airports and airlines as to what the airline pays to use the airport. The airline influence over the PRM element of costs is probably the lowest of any component of the fees and charges paid by any airline at any airport. The Manchester PRM charge per departing passenger is already towards the upper end of the scale for major airports (although not the highest). It doesn't seem to correlate to the service being provided also being at the upper end of the scale. Either way, the ball is firmly in the airport's court to fix it.