Cancer, in particular, can be practically free to insurance if caught early. Colon and skin cancer are the poster children. Colon cancer can be treated in the process of doing the screening when caught early. And skin cancer is a pretty minor "just lop off that mole" procedure that also ends up being the treatment.
Letting it grow and catching it when symptoms arise is terribly expensive. The chemo, surgery, scans, and frequent doctors visits are all crazy expensive.
About the only way I could see preventative care not costing less is if you just let the people die and call it god's will rather than calling it a death that could have been prevented.