Your Money/Time principal has long been defined. It’s called Opportunity Cost. You have experienced this principal your entire life. In many cases, especially as we earn more, this principal tends to involve money. However, it’s really about value and the decisions we have to make in life.

As far as how you express it, your conclusions are a bit off. The method by which you earn money (hourly, salary(taxable or not)) has no bearing on the value traded. Additionally, both time and money are finite, therefore a tradeoff must occur. This is true no matter your ability to work more or you’re a billionaire. In your approach to this idea, you’re assuming some kind of concrete, always balanced, equation. While this is more true(and easier to see as balanced) when opportunity cost is applied in business (outsourced factory can make widget X in bulk cheaper), this is almost always false in life situations like you describe. There are just too many variables. Skill, willingness, enjoyment, or necessity as just a few of the intangibles that factor into the decision.