It cost one large bank in Australia $1 Billion over 5 years to convert their system. That's 2-3% of their annual profit over 5 years to fix the problem, so we c

It cost one large bank in Australia $1 Billion over 5 years to convert their system. That's 2-3% of their annual profit over 5 years to fix the problem, so we can assume the costs of running the current system are probably some small multiple of that. And they would still need to pay to run the new system, and most likely both systems at the same time towards the end of the transition.