I've worked at many big banks and corporations. They are all held together with the proverbial sticky tape, bubblegum, and hope.

They do have multiple layers of redundancies, and thus have the big budgets, but they won't be kept hot, or there will be some critical flaws that all of the engineers know about but they haven't been given permission/funding to fix, and are so badly managed by the firm, they dgaf either and secretly want the thing to burn.

There will be sustained periods of downtime if their primary system blips.

They will all still be dependent on some hyper-critical system that nobody really knows how it works, the last change was introduced in 1988 and it (probably) requires a terminal emulator to operate.

I've worked on software used by these and have been called in to help support from time to time. One customer which is a top single digit public company by market cap (they may have been #1 at the time, a few years ago) had their SAP systems go down once every few days. This wasn't causing a real monetary problem for them because their hot standby took over.

They weren't using mainframes, just "big iron" servers, but each one would have been north of $5 million for the box alone, I guess on a 5ish year replacement schedule. Then there's all the networking, storage, licensing, support, and internal administration costs for it which would easily cost that much again.

Now people will say SAP systems are made entirely of dict tape and bubblegum. But it all worked. This system ran all their sales/purchasing sites and portals and was doing a million dollars every couple of minutes so that all paid for itself many times over during the course of that bug. Cold standby would not have cut it. Especially since these big systems take many minutes to boot and HANA takes even longer to load from storage.