On average, how much does each individual customer cost a bank to serve?

This is a very complex question with no easy answer. Vendors/solution providers provide varying discounts in each geographic territories to be able to close business with each bank. Finding out how much a bank has actually paid for the software (like Core Banking) would not be an accurate enough mark, even if you were to somehow obtain that number. Ancillary costs must be taken into account, software maintenance, HR, implementation, regulator's audit, transitioning, network connectivity, hardware, support hard (like UPS, generators, fire suppression, etc.) networking hardware, Database (and all costs associated with the database, etc.)

The same can be applied for ATM transactions, Mobile Banking, Internet Banking, Etc.

Each country has its own set of pricing mechanisms. Some like the US, do not give a steep discount, other markets like the Scandinavian countries, rarely get discounts and they believe in paying premium, which includes premium human resources as well. Middle East varies, South Asia relies on deep discounts, South East Asia, HR is expensive if you look at countries like HK or SG, whilst PH, IN, ML are relatively economical. The network costs again depend on the carrier, the population density, the metrics at play within each economy

You will find various vendors publishing their reports, but you must understand the pricing averages reflected in such reports are for their own walled garden. Even the likes of Gartner et. al. don't really get the true pricing figures (they are pretty close, but nonetheless still estimates).

Your best bet of finding out transactional costs for certain elements as you have asked above, is to look at the transactional banking figures for the economy published by the Central Bank, they might give you some form of rough semblance of how much an ATM transaction, or the average amount processed from an ATM (though that isn't exactly what you are asking).