And then there were the software licences and cost to develop software.

As a rule of thumb you could calculate those like: PC (1) - minis like e.g. AS400 (multiply by 10-50) - mainframe (multiply by 100+).

For example in the early days of MS there was an MS Excel running on IBM mainframes - US$200K per license and per year.

To top that when IBM, DEC e.a. where among the few providing such computing power (into the 1980s) and those companies that required such bang on a broad scale - banks, insurances, research institutions but particular banks with their need for many workstations as well - IBM did framework contracts forcing their mainframe customers to also buy their PCs as workstations often for many years.

That was one of the reasons why you found all those very expensive IBM PCs in banks - tens of thousand in each of the larger banks - while you could already buy no-name PCs for a fraction of the price. (A fully equipped PS/2 could easily cost the price of a small car and for development machines you could get up to a Mercedes S-class entry level price - by now prices of the PCs dropped to a fraction while the prices of the cars multiplied by 3-5)

An interesting example of how monopolies / oligopolies are created / kept alive / and it still takes many years in markets moving as fast as the IT industry until they die or loose the grip on their customers.

But then you'd have to reinvent mainframe reliability and IO capacity out of commodity hardware and software.

I'm not saying it's impossible. It's just hard. Maybe even US$ 700,000 hard ;-)

... And then spend the same again on fabric to support that much I/O. Supercomputers/clusters for compute, mainframes for IOPS.

"A supercomputer is way to turn a CPU bound problem into an IO bound problem" as the saying goes.

Well, sort of. That's why I said apples/oranges. Thing is, for that kind of money even a nice FC/IB-fabric is in the budget - perhaps only every 2 months then.

Of course there are latency tradeoffs, no idea what the z9 was used for. But no matter how you spin it, $700k/yr is one hell of a TCO for a piece of hardware that is almost 10 years old.

The z9 is about 6 years old at this point. At the time 512Gb RAM in pizza boxes simply didn't exist.