I think the “time is money” metaphor is very useful to thinking about time as a limited resource. The average person has about 630,000 hours in their life. Take out 210,000 for sleeping, a largely involuntary time sink. The time cost is truly the more expensive portion. This limited resource consideration is even more important to consider there are strict upper-limits to how much time someone will have, and no one has lived 1,000,000 hours.
I appreciate you say this with humour. But if you voluntary or due to circumstances out of your control cannot distract you brain... At least nine starts thinking. The energy cost is high, but I always end up with a very profound experience and some great outcomes.
So I wouldn't cross out "staring at a wall" as inarguably invaluable. It's what's going on in your head when you do it. Same as when reading that book.
I don’t think it’s just time but also, as you allude to a bit, the extra effort, mental or physical.