What stops a club from putting an absurd release clause amount on a
players contract?
Nothing, except the player himself (or his agent).
The value of the release clause of a player is proportional to the unwillingness of the club to sell the player in the future. For example, in an interview to the BBC, Cristiano Ronaldo's agent Jorge Mendes said that Ronaldo's release clause from Real Madrid was 1 billion euros! Which essentially means that at the time of signing that contract neither Real Madrid nor Cristiano Ronaldo were considering any possible transfer negotiations with other clubs during his contract.
The release clauses can be absurdly high, but then if the club declines any transfer offers (despite the will of the player) below the release clause, then the players that wanted to be transferred may get angry and wait till the end of their contracts and then would go for free.
The less years there are left in the contract of a player to play for his club, the less is the power of the release clause. Why? Because, if a club declines transfer offers below the release clause when the player's contract is due soon, eventually, a player may leave his current club for free when his contract will expire.
The rule of thumb for the club is when defining the release clause is to make it high enough, in order to create a room for possible future transfer negotiations, but also keeping in mind that players' agents will not agree to the extremely large release clauses if the player himself may consider in the future leaving the club during his contract.
