>OK, let me make it simpler. To provide a software service to a customer costs $X per month in costs
>(that's running costs and maintenance costs and lets say enhancements too).
>In your way thinking what do you charge the customer to provide it?


If you are providing a service to that customer to alter the software the way they need it to be altered, and they agree, then sure. It's an agreement for the service you provide. Charge them $X per month and agree with them for some reasonable amount above that to cover unexpected expenses.

All that you agree to is fine. But all agreements should be tempered, with their focus being on service and help and growing people up together, and not just on raw money making abilities because you happen to have a niche market, for example, and can charge 5*$X per month and people will pay. All such charging is wrong.