How much money you save your users can be automated if you collect data about common transactions with your service.

If your users save money by borrowing things instead of buying, you can calculate the amount saved per transaction, using historical data.

If your users save money by using a service from you instead of another, more expensive place, you can calculate the savings if you know the difference in price.

For every calculation, think about the confidence you have in your data, and factor in a range for the amount of money actually saved.