Well, those questions depend on the system in question, and for any particular system, it's probably impossible to say what the cost of replacing would be, because no one understands everything the system does, and how it interacts with external systems. This is what others are alluding too, your question is unanswerable.

What you can do is estimate the cost of implementation in today dollars. For example suppose Bank System X has been under development for 40 year with an averge of 10 developers per year. That's 400 person years. Suppose you find cheap labor at $100k per year, the sunk cost in personnel alone is $40M in todays dollars. So 40 people maybe could do it 10 years.

The above analysis is not satisfying.