That's not how it works. It depends on whether (how much) they are paid and if so what do they buy with that payment. Are they settled in US buying pizzas and Netflix?
Because if not, then investments to Nigeria past $x will grant it the upperhand since all that money can get you another doctor and more.
5 years is truly a big kill. looks like its a reaction to 99% of them refusing to send money back home and invest in Nigeria, preferring to buy-US and -Europe.
Not sure if you are deliberately misunderstanding at this point, but the problem is obviously not where the doctors spend their money, but the fact that they are not practicing medicine in Nigeria – as Nigeria might reasonably have hoped them to do once they completed their training.