It can be higher than the salary, however it gives an option for it to be challenged in court as it's unreasonably high.
So the Real Madrid buyouts could very well be true. Dembele has a higher buyout than Messi, Suarez and others, and on less salary so there you have it.
I think in footballer case the purchase prices counts also, so a player like Messi can't have a bigger buyout than Bale/Cr7 or Dembele. But I do believe we aren't putting the maximum buyouts in all cases.
Doubt RM buyout is true, there is no evidence but some stupid reports about it. And almost all other clubs in Liga has more reasonable buyout, even Atletico Madrid.
I would say we need to put maximum buyout in all players and no "if played less than X minutes it get lower" clauses. But we can't put billion in buyouts or expect Barca B player's buyout to be huge.