In 2006 IBM introduced the concept of a PVU, the idea being that all processors are not created equal and thus a Power-8 core is capable of doing much more work than an Intel Itanium. The PVU value of your system is then used to calculate the number of software licences you require.
This table provides a list of the power-ratings of each processor-type and enables you to calculate the PVU rating of your system:
Whilst this is a very handy reference I would still recommend that you contact IBM directly in order to negotiate a discount. Remember that you can also use virtualisation and workload management technologies to control the allocation of your resources in order to save money.