To quote a cartoon " welcome to the wooooorrrllldd of yesterday!!" or something like that.
The problem with System Z is it varies wildly in price and performance. And more so than any other platform, You need sysadmins (sysprogs in the Z world) that know what they are doing. I can be a complex environment and its quite different than any other ( in my opinion).
That said, from the sysprogs that ive got around you might be better to look around for second hand mainframes rather than buying, because the hardware is still really good, but much cheaper. checkout http://www.trustnsr.com/used-ibm-mainframe.html for example ( Im not endorsing them, just giving you an option, they were first on a google search). I heard that you can get the lowest end z10 for 50k. But you really need to have a look at the rest of the infrastructure needed. I get the feeling that that is just the tip of the iceberg. You need a proper datacenter, with seperate storage,yada yada yada.
Have you thought of looking into outsourcers? there is plenty of them all over the world, and it's probably better than buying one, AND hiring a Sysprog to maintain it.
Of course the cost is offset by the eventual huge profit margins that mainframe software attracts, so thats always a positive.
Btw I world in a mainframe lab with about 150 devs though I am one of the younger ones here, just to let you know i do have some experience, not a lot tho.