The charging structure depends on the vendor that you are buying your CPU time from. Typically charging is based around MIPS, memory, and storage. Time of day can affect the rates too. But there is no standard that I’m aware of.
As to finding out what you need to buy to improve performance, your best bet is to talk to IBM or your favorite IBM reseller. Memory may improve utilization and performance (depending on how much paging is going on) but it will not increase the MIPS of the machine. Again, talking to IBM or reseller is the best thing to do.
The other thing you can do is go to a user group meeting such as SHARE (www.share.org) where you can interact with technical peers who have faced the same questions and can talk to you about what they’ve done.