At lower payments I agree that there is a big difference and that people pay too much for potential. But it is mainly newbies that makes this mistake and they have not got enough money to spend $500000 on an old player just because of potential. Even if this user obviously overspent on a high potential player I very much doubt that it is that common that someone will pay as much as $500000 for a useless MVP potential as his player actually is with that age and salary.
And I agree that people overpay for potential. That is the biggest mistake new users does as they tend to believe that potential trains players faster already from start.