Software emulation is fast enough?
Tom Lehman, founder of TurboHercules (a Mainframe emulator that runs on x86) writes
"...we can run a reasonably sized load (800MIPS with our standard package). If the machine in question is larger than that, we can scale to 1600MIPS with our quad Nehalem based package and we have been promised an 8 way Nehalem EX based machine early next year that should take us to the 3200MIPS mark. Anything bigger than that is replicated by a collection of systems."
So if you have a need of something like 3000 MIPS, then you should use a x86 server and emulate a Mainframe in software.
How much does a cheap Mainframe cost? 1 million USD? What is the performance? 100 MIPS?
How much will a PC with Nehalem EX cost? A couple of thousand USD? What is the performance? 3.200 MIPS
You make your choice.