As the maturity of service management increases, it is possible to deliver higher levels of services without a proportional increase in costs. How?

It is possible if the maturity lead to eliminating waste in the service management processes. This will increase the level of service without increasing cost, and it might reduce the incurred cost.

I believe that quality comes with the price. As the service management reaches the stage of maturity, it sets the quality standards of the processes in line with the organisational creditbility. To maintain the highest level of service you need the quality resources which comes with a price but at the same time you can reduce the cost by eliminating the loopholes from the processes.