I should look up that one because I recall it being said once that this was actually scientifically proven.

Not the exact but you can glean the idea: https://www.researchgate.net/publication/19416122_Money_versus_pain_experimental_study_of_a_conflict_in_humans

From what I recall, and assuming that's correct, volunteers were tested for their endurance to pain/discomfort via a hot plate. First they were tested as normal. When being told to just hold a significant amount of money, their tolerance increased slightly, and when told they could keep the money, it increased dramatically, to the point that, with enough pay, they could endure the maximum temperature they were allowed to set the plates. (around 50-55C/120-130F).