Quantifying effort isn't really feasible, but incentive based, partially non-guaranteed contracts are essentially what US sports run off (especially contact sports) so I don't know why Australian rugby players should somehow be above it
The only US sport where contracts aren't guaranteed is the NFL and that is moving towards far more guaranteed money, particularly for the best players.
Like all sports the best players get paid the most money, get the most guarantees and get the most favourable contract terms (such as no trade clauses etc.).