I don't know if there are studies that compare those opportunity costs directly, and to me that seems like a tough experiment to set up/run properly.

I seriously doubt, however, that that is an actual trade-off being made in real life. As in, no one I know or have ever met is _actually_ making that trade-off. I've known people that claimed similar reasons for not exercising regularly, but the reality is that they just don't want to -- not that they are making a conscious trade-off based on opportunity cost.

Reality: Most people are probably better off taking 1-1.5 hours of low quality work/fucking around on their computers and use it to exercise instead.