I think some of you are missing OP's point. He is saying that there are significant opportunity costs ("diminishing returns" isn't the correct term here for what he's trying to say) to going into medicine since what is valued in the process is unfortunately not high paying and that that there are high barriers to entry in the field. Most undergrads and medical students probably wouldn't do any academic research if it were not highly valued in the medical school or residency application process due to little or no pay (even if it's try that they don't bring much to the table, but they are still forced to do it), and this was the case about 30 years ago where even those going into competitive specialties did not do research unless there were very passionate about it or were dead set into going into academic medicine. Also, stuff like volunteering and shadowing in undergrad that premeds have to do are also unpaid, while compared to other fields such as finance, where internships with financial institutions are valued as part of getting entry levels jobs but are much better paying (many undergrads can make $20-30k per summer with investment banking internships so even accounting for the long 80-100 hrs of work per week that is still much better paying per hour) and the upfront investment in both time and money is significant.
Also, the difference is stipends that people mention here probably accounted for in large part by the differences in cost of living.