Source?
They're taking maybe $1-3 million less this season, but if they make it to a second contract in the NBA the amount they'll gain by having gotten there a year earlier will likely eclipse the loss. And there's no guarantee that draft stock doesn't go down after another year in college. A player hypothetically could play himself out of the draft entirely or suffer a serious injury, thereby costing himself tens of millions in lifetime earnings. Jayden Quaintance probably would have been a top 10 pick, maybe even top 5, after his freshman year, but he was too young to declare. Coming back to college probably cost him between $10 and $15 million on his rookie contract.