This is a narrow view of the “opportunity cost” problem.
In practice, yes, the ore you mine is yours. It has value, you have been paid for your time. If you are happy mining. I am not going to deny you happiness.
If you are looking at it from a purely wealth point of view, then you could have done another less fun task, earned more that way then bought the ore from someone else and still had a some cash left over.
And who am I to deny you the right to be rich and miserable?
Every Opportunity has the cost of “by doing X I didn’t do Y”. And there are many measures of value: enjoyment, ISK, reputation. They are all trade offs.
As a relaxing bit pf past-time I like mining - a gentle packing/optimisation puzzle. I am fully aware that I could earn at a faster rate doing abyssal runs, or taking the exploration ship down the layers of wormholes. But on occasions, I just like the contented feeling of a well packed ore hold.
The choice I make, and the Opportunity Cost I am willing to pay for making that choice is something I fully understand.
Of course, Economics (that old dead hand) is all based around monetary value rather than emotional or other values. But there you go…