The role of the wife to the heir isn’t a ‘job’. It’s a position.
The term working royal refers to members of the monarch’s family who are designated official representatives of the Crown.
They don’t get paid a wage in the same way a regular job is remunerated.
The heir and his family are funded by the Duchy of Cornwall.
The other working royals - are funded by the Duchy of Lancaster via the King. He decides how much they get paid.
Travel expenses for official duties are paid via the Sovereign Grant.
How many engagements or what type of engagements are done by any particular Royal are ultimately the decision of the monarch - as they are his ‘team’ to deploy.
He is also taking instruction from the government of the day on which specific constitutional and foreign policy commitments they expect from the head of state.