It seems to me we have three basic things to manage: money, time, and energy.
Money is the most straightforward. Because we know we will run out of time, and we may or may not run out of money, it seems that time will ultimately be the more precious resource. Thus, trading money for time seems like it would usually be a good trade if we have the option.
Trading time for money (working) is a necessity for most of us, but in the end is not a good trade. What good is money when you have run out of time? Or what good is money if you have no time to spend it? Perhaps once we pass a baseline of sufficiency for money, we should be open to trading that for time.
Energy and time almost seem like the same thing, but I distinguish because I think you can have time and yet not the energy to use that time for something worthwhile.
For example, at the end of a busy week of work, you may have a few hours to manage. If your willpower is depleted, then you will be unable to spend those hours in the way you might like to (say, working on a side project).
Thus, in order to spend time well, you need not only to have the time slot open, but you need to also have a reserve of energy to use. This energy is something we manage in the same way that we manage money and time.
Perhaps energy is more similar to time, because it has a fixed limit, whereas money does not (or at least, money is not tied to a personal limit).
Or does energy have a fixed limit?
This brings us to the value of discipline. Discipline is something that is painful in the moment, but produces value in the future. Self-discipline is the capacity to choose to do something that is painful because you know you want the value it brings.
It seems that this capacity is not fixed, but rather elastic. It can be strengthened by exercise, like a muscle. Discipline is precious because it enables us to spend our time in a way that can see beyond present pain (more efficiently). Discipline is also something we can choose to invest in.
If you start with the idea that time is most precious of the three resources, because it has a fixed limit that we have little to no control over, then the question becomes how do you spend money and energy to get more time. You can spend money to save you time. Investing in discipline is a multiplier of time, because it helps you spend the time you do have well.