From Seneca's Letters From a Stoic, Letter 42, "On Values":
Therefore, with regard to the objects which we pursue, and for which we strive with great effort, we should note this truth; either there is nothing desirable in them, or the undesirable is preponderant. Some objects are superfluous; others are not worth the price we pay for them. But we do not see this clearly, and we regard things as free gifts when they really cost us very dear. Our stupidity may be clearly proved by the fact that we hold that "buying" refers only to the objects for which we pay cash, and we regard as free gifts the things for which we spend our very selves. These we should refuse to buy, if we were compelled to give in payment for them our houses or some attractive and profitable estate; but we are eager to attain them at the cost of anxiety, of danger, and of lost honour, personal freedom, and time; so true it is that each man regards nothing as cheaper than himself.
This reminded me of this scene from The Fountainhead:
“Howard—anything you ask. Anything. I’d sell my soul ...”
“That’s the sort of thing I want you to understand. To sell your soul is the easiest thing in the world. That’s what everybody does every hour of his life. If I asked you to keep your soul—would you understand why that’s much harder?”