I haven't worked with mainframes since the z10, but back then you could get into an entry model for about $100k.
Though the sky is the limit. The typical machine I would order had a list price of about 1 million. Of course no one pays list. Discounts can be pretty substantial depending on how much business you do with IBM or how bad they want to get your business.
It depends on how full those drawers are. $250k to $1m would be the typical price range.
It's easier and harder at the same time to buy older hardware. That's half the challenge though because the software is strictly licensed and you pay per MIPS.
Here's a kid who bought a mainframe and then brought it up:
The big problem is that everything in IBM-z world is negotiated, and often covered by NDAs. The pricing is complicated by which operating systems and what sort of workloads you'll be running, and what service level guarantees you need. The only published pricing in the entire life of the IBM 360/370/390/z-series line was the Linux One when it was first released... Hardware plus OS, excluding storage, was $70k on the low end.
Previous generation machines that came off-lease used to be listed on IBM's web site. You could have a fully-maxed-out previous-generation machine for under $250k. Fifteen years ago I was able to get ballpark pricing for a fully-maxed-out new machine, and it was "over a million, but less than two million, and closer to the low end". That being said, the machines are often leased.
If you go with z/vm or z/vse, the OS and softare is typically sold under terms that are pretty much like normal software, except it varies depending on the capacity level of the machine, which may be less than the physical number of CPUs in the machine, since that is a thing in IBM-land.
If you go for z/os, welcome to the world of metered billing. You're looking at tens of thousands of dollars in MRC just to get started, and if you're running the exact wrong mix of everything, you'll be spending millions just on software each month. There's a whole industry that revolves around managing these expenses. Still less complicated than The Cloud.
Hercules is _not_ used by IBM's own developers. Being found with Hercules on your computer at IBM gets you in trouble. I know people who work on mainframe-related stuff inside IBM and they steer well clear of Hercules. And I've heard that IBM's computer monitoring stuff (antivirus, asset protection, etc.) looks for Hercules and flags it.
But IBM _does_ have their own mainframe emulator, zPDT (z Personal Development Tool), sold to their customers for dev and testing (under the name zD&T -- z Development and Test), and to ISVs under their ISV program. That's what IBM's own developers would be using if they're doing stuff under emulation instead of LPARs on real hardware.
(And IBM's emulator is significantly faster than Hercules, FWIW, but overall less feature-full and lacks all of the support Hercules has for older architectures, more device types, etc.)
There was some of a legal fight between IBM and Turbo Hercules SSA, a company that tried to force IBM to license z/OS to their users. IBM has been holding a grudge ever since (probably at the advice of their legal).
You can run the emulator but you will not get your hands on
new versions of the operating system to run on it.
But there are old versions that you can get your hand on.
You don't buy a mainframe, it's consumption based pricing. They aren't just going to list a price, because they need to size the hardware to what they think the workload will be.
Could they just list prices? Sure. Will they ever do it? No.
It's probably impossible to say because of the service contracts that come with it. Nobody would buy one brand new and not pay for support and consulting too.
Though the sky is the limit. The typical machine I would order had a list price of about 1 million. Of course no one pays list. Discounts can be pretty substantial depending on how much business you do with IBM or how bad they want to get your business.