Every process improvement initiative would have some investment that you need to make in order to get the process more mature which inturn would save costs . So, it is important that the actual costs saved consider the investment you have made in the improvement. Usually you use the ROI ( Return on investment) to calculate the improvement you have achieved . For banks back office process ( or any process for that matter of fact) the cost of running the process which would include the infrastructure as well as resource cost needs to be worked out and then post the improvement the savins in either the infra cost or the resource cost which is usually measured as a productivity improvement can be calculated. Hope this helps address your concern