They happen to be two-fold. There’s the opportunity and there’s the cost.
The opportunity is a chance to spend our time in another way. And the cost is the money we may (or may not) lose out on by spending our time in the way we’ve decided to.
If the opportunity is worth it, the costs may not even matter.
I think the real question is, what do we give up by overthinking the costs and undervaluing the opportunity?