Treatment
Its difficult i'm afraid. If you use the case of saving staff costs due to say more rosterers required it won't add up as a system costs a lot more than one or two extra staff. There's also no guarentee the system will do things quicker, AIMS for example strugles to autoroster . The only way you can argue it is against slightly lower crew numbers, they are more expensive.
You might not actually be able to save any crew, get the system in then worry about that.