Are you removing paid working time in doing the extra work? If not it is just an opportunity cost

In my line of work, yes.

This is a fairly crappy methodology though because the vast majority of people are not substituting paid working time with researching buying things online. So it hasn't "cost" them anything other than their free time which is far more complex than an hourly rate. Maybe they enjoy researching products and in that situation, it wasn't a waste of time at all.

Exactly. I always hate these "your time is worth your hourly wage" arguments. They're often used to argue against things like changing the oil in your car, fixing a clogged drain, or DIYing anything.

Your time is only worth money if you'd otherwise be working at that rate, which is not the case for the vast majority of humans.

Saving money is the same thing as earning it. Are you better off spending an hour to save $100 or an hour to save $10?

I once spent 2 hours negotiating the purchase of a car. It saved $5000. That works out to $2500 an hour. Was it worth it? Hell ya!

I've also worked hourly jobs in the past. There were often opportunities to work more hours or overtime. People often have side hustles, too.