Comparing how far below what a NBA star should get vs what an NFL star should get is impossible.
NFL stars have no max per year once they get past their first contract, outside of the overall team max salary cap. A cap that has to be split between 53 players.
NBA stars salaries are artificially limited to a % of the total cap. It's why so many of the mid to high level players are just given the max and almost no holdouts in the NBA.
If anyone wants to talk about true underpayment, it has to absolutely be Lebron, and really any of the top 5 of so NBA stars. I bet Lebron on an open market, not restricted by individual player % of the cap, would be offered up 90 million if an owner knew he could get him and fill the rest with vet mins, exceptions, and draft picks.