(With Unisys specifically, at one point they still made physical CPUs for high end models, but low end models were software emulation on x86; I’m not sure what they are doing right now.)
It was my first exposure to this sort of thing, and I was taken aback by the costs of this stuff, which made the Sun gear I worked with look extremely cheap :)
Given the shrinking market share of mainframes, the only way for vendors to continue to make money is to increase prices on those customers who remain – which, of course, gives them greater encouragement to migrate away, but for some customers the migration costs are going to be so high that it is still cheaper to pay megabucks to the mainframe vendor than do that migration. With emulated systems like the ones you saw, the high costs are not really for the hardware, they are for the mainframe emulation software, mainframe operating system, etc, but it is all sold together as a package.
At least IBM mainframes have a big enough history of popularity, that there are a lot of tools out there (and entire consulting businesses) to assist with porting IBM mainframe applications to more mainstream platforms. For the remaining non-IBM mainframe platforms (Unisys, Bull, Fujitsu, etc), a lot less tools and skilled warm bodies are available, which I imagine could make these platforms more expensive to migrate away from than IBM's.