So Republicans don't @ me for the following - I know that those who don't want a monarchy at all consider both Duchy money as 'taxpayer' money but in the following I'm talking specifically about current direct taxpayer contribution and the concept of the monarch and heir (and other working royals) being a 'job'.
The only direct taxpayer-paid money from Government to the Monarchy is the Sovereign Grant - which is used for the upkeep of the Crown estates which are kept in trust for the country, expenses used by royal family members when they are on official duties, and all the official receptions, investitures, garden parties etc.
If money isn't spent during the year it goes into a reserve fund which is monitored and controlled by the Royal Trustees which is the PM, the Chancellor and the Keeper of the Privy Purse.
So PoW or the heir to the throne if not the PoW is less a job and more of a position. The only actual requirement is to stay informed and ready to become head of state, and while waiting support the current head of state with any duties the monarch needs you to perform.
So if William takes a month off, he's not receiving a taxpayer salary for not doing public engagements, in fact he's spending less of the sovereign grant in doing so.
The SG's business accounts are monitored and audited by the National Audit Office and laid before Parliament.
I haven't looked into how detailed those accounts are or whether they are available in detail to the public but that's what I would like to see as a process of more accountability. To see exactly how much money is spent on preserving the Crown assets vs investitures, vs official receptions vs working royals travel and expenses etc.