So imagine an environment where you can't buy your hardware, you can't buy your software, and when you've made an upgrade to your hardware your software gets even more expensive. That's a mainframe.
Our mainframe, which is mostly used for customer billing (electrical utility billing is slightly complex due to all the different tariffs, localities and taxes), and customer relationship & service management, is costing us $15 million - $20 million per year. On the one hand you'd say holy smokes that's expensive! But on the other you could say that's only $3-$4 per customer per year. Thing is you could do the same amount of processing in AWS for under $250,000 per month or $3 million per year, making the mainframe at least 5-7 times more expensive than AWS.
So how does saving $17 million per year justify spending $1 billion?
Is that worth $1 billion? That's above my pay grade and I'm glad I don't have to make that decision. For the record, our CFO doesn't think so - at least not until he's closer to retirement so if he gets canned for it he'll be alright! He's in his mid 50's now so we may have to wait another five years!