I think if the op was talking about non-monetary costs.

But if we're limiting this to monetary costs only, I believe you're right. I saw a similar study from the UK circa ~2008ish that compared long term smoker/obese/healthy medical costs, and healthy people are indeed the most expensive (mostly because of tremendous end of life care costs).

Edit: I dug around a bit and found the study I was thinking of. It was actually a study on obesity (also not done by the UK), but used (non-obese) smokers as a group for comparison.

Link to article: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2225430/

See Table 1 for lifetime medical costs.

This has never made sense to me. I'd be interested to know if there are numbers to back it up. You still have your most expensive years (the ones that come before death), they just come much sooner and are much more likely to involve expensive debilitating disease. In any case, the expense of old age mostly comes from pills and surgery which are artificially expensive due to our shit healthcare system. And I doubt we accurately measure the economic benefit of having the elderly around to help with young families, provide wisdom and experience, etc.