I have in the past developed cost-savings calculators and they can be one of the most effective tools in moving an enterprise software prospect through the sales funnel to a closed deal. So I understand why you are so keen to get a handle on these numbers.
Unfortunately, you are in a bit of a chicken-and-egg situation. It reads as though this will be your first deal. By definition, then, you have not yet had a deployment allowing you to properly quantify and document the benefits and cost savings accruing to insurers when their patients use your tool. Without such data, on the other hand, closing this first customer is all the more difficult.
What you do have is a golden opportunity to work with this early adopter of your technology to develop those metrics. Make it worth this customer's while to work with you by offering it a steep discount, or a modest up-front fee combined with payments down the road based on how much they actually save. Or collaborate with this customer on a study that will quantify such savings and will provide this insurer with some effective competitive positioning it can use in its own marketing.
The challenge here is that savings on the mediation of lifestyle diseases like diabetes can take an awfully long time to accurately establish. What metrics, such as weight stabilization, could be harvested in the shorter term? And how convincingly could you extrapolate from those early metrics to estimate eventual cost savings?
You are certainly thinking the right way. Now you just need to figure out how you can collect actual patient data and turn that into a cost-savings calculator that will help you more easily close the next customer, and the scores of customers thereafter.
I hope this helped. If you have further questions, I am only a phone call away.