Also the per-core licence is not the only position. You have to add a maintenance contract, buying the machine, firmware licence (which in this case is afaik a big Java application running on its own core), OS licence, setup, and probably more.
This really is a cashcow of IBM, mostly because they own the "mainframe" trademark/monopoly and everybody who wants to sell a "mainframe" is sued out of the market. Without the "mainframe" title, you are dead in this market of ultra-conservative customers (banks,airlines,etc).