The National Health Service Corps (NHSC) Scholarship Program and Indian Health Service Scholarship Program will pay for medical school in exchange for agreeing to work in underserved areas for several years. Some states have similar programs. I'm not sure how you would even begin to calculate ROI for that.

https://www.congress.gov/crs-product/R44970

It's complex and imperfect, but in this situation the most direct ROI would be the cost of recruiting a newly graduated doctor by increasing the salary until someone accepts compared to the cost of providing the scholarship.