£50m even without add ons is very much what Antony is worth max.
It may be a realistic value, but Ajax can sell him for that value next season even if he doesn't have an extraordinary season. However with another opportunity for him to shine in CL and a WC coming up and since he has all the talent to make an impact at any level, he can easily have an increased value next season. Since Ajax don't want to sell right, he has 3 years left on his contract and there's a good possibility his value will increase (significantly) in the next 12 months, Ajax are not selling for a reasonable fee.
Besides that, as was discussed earlier in this topic, Richarlison and Raphinha really aren't better than Antony and they are priced at £60-65m. Antony is younger and has a higher ceiling. They are not better players just because they have played in the Premier League before. Ajax would look like total idiots selling Antony for £50m at this point in time.
I can see Ajax moving down from the £68m asking price, but it's going to take £60-65m to get Ajax to sell. I agree this is a steep fee at this moment in time, but Ajax hold all the leverage. There's no reason for Ajax at this point to sell for a reasonable fee. Also has nothing to do with United showing interest, which is what some people suggest. It's just business. Ajax don't want to sell this player, the player has a long term contract and Ajax rate him very highly and have all the reason to believe his value will increase in the next 12 months. In this situation it is simply going to take a steep fee to get the deal done. There is no reason why Ajax should accept a reasonable fee in this situation.