I'm reminded of the law of diminishing returns.
The idea is the more you put into production, the less you get out of it. I originally stumbled upon this theory while looking for a mathematical model to model the increased demand for addictive substances, but noted the harmonic series https://en.wikipedia.org/wiki/Harmonic_series_(mathematics) also modeled it, where the second chocolate bar, cigarette, or dose of heroin is only half as good as the first, the third even less, the fourth, etc.
But more salient to what you are looking for, investment into graduate school, in terms of time and money, naturally comes with greater cost, compounded by that diminished return on investment lost by decreased life time earnings in terms of raw years. This could be applied to the "worth of skill" in terms of enjoyment of a highly skilled musical instrument player vs. a skilled musical instrument player, as the number of pieces of music and number of people you can impress first opens very wide, then the limitations become more precise, and the scarcity of musical challenges increases so that a finer and finer list of musical rivals and musical challenges exists. With respect to both a musician and a graduate student, or a marathon runner, in addition to losing the time invested, there's also competition.
Rising in a given devotion forces you into the category of competitive X, such as competitive gambler, competitive skier, or competing against other graduates for the same job. This manifests either as literal reduction of wages, being over qualified or having to compete with a dozen other Ph.D students or a hundred scientists seeking the same grant money. That may be counter intuitive at first, but because the demand for extreme specialization decreases with the increased level of specialization, the supply of rewards for that increased specialization also decreases. With gambling or cheating gamblers, the risk continues to increase and the skill of rivals increases to to financially dangerous levels, as notoriety results in an endless sequence of people actively trying to bankrupt or arrest the gambler. Bank robbery theory is the same way, as criminals often talk about "just one more job", assessing their risk vs. rewards and concluding (usually after arrest) that they should have quit while they were ahead.
Opportunity costs are extreme for some of these models too, as a skilled musician, for example, may impress at 3000-5000 hours of practice, but need 7000-10,000 hours of practice for the average person to appreciate any difference, and 20,000 hours of practice for a subtle improvement beyond that. With a marathon runner, the more you practice, the older you get, and the closer to "peak" you reach, and the more frequent the opportunities for injury, such as the recent Usain Bolt run. Boxing has a similar diminishing returns effect as injuries compound and performance decreases - that was a central theme of Rocky. Football Salaries are exceptionally high with a similar peak and injury calculated into the risk assessment.
Diminishing returns are most poignant though with the college graduate. You are assumed to have X number of years working, and every year in school is a year where you not only are not earning optimal wages, you are also building debt that will compound. Since work experience also increases wages with time, and since student loans have interest rates, there is a point in which a person's life time earnings actually start to drop lower. At one school I was attending, the tuition total was around $50,000/year, and to complete everything through graduate school was going to total half a million dollars and take a total of 10 years. As starting age was a constant and the growth of returns depended on the time from graduation to the time of retirement minus the growth of loans, the model for staying didn't pan out, and quitting was more fiscally sound.