There is something that makes me skeptical about "exercise" considered as an abstract quantity. There are many different ways of doing "exercising", and they have many different effects on the body. How much exercise you do (but then, counting in hours ? spent calories ?) do matter, but in which conditions also matters a lot : how regularly, what kind of exercise, ...

Exercising usually is good for urban sedentary people who "naturally" don't do much of it, but it can also have negative side-effects, in accelerated aging or articulation damage. Professional sport players have a very short average lifespan, and it's not only because some of them use drugs.

There is surprisingly few literature on the topic (or I couldn't find it), but from what I remember finding, something important is the regularity (doing 3 hours of exercise twice a week is not nearly as good as doing 1/2 hour each day, even if 1/2 hour each day is just 3.5 hours in total) and the kind of activity : constant medium/low intensity, long duration activity (walking, long-distance running, bicycling or swimming at a moderate speed for a long while, ...) is much better than variable intensity (like ball games, where you run and stop and run again and kick and ...) or high-intensity short-duration (like sprinting).

As for opportunity cost, something to consider is doing exercise that fits a purpose, instead of just "wasting" hours doing it, for example, running/walking/bicycling instead of using a car means you do some exercise but also save on money and on accident risks, especially if your environment (public transport network, proximity of things, ...) allows to not own a car at all (making the money gain very significant), or finding an activity that you actually enjoy so the time isn't fully wasted.