What most people refer to as the taxpayers money - the Sovereign Grant - which is a percentage of the profits from the Crown estates is monitored by government oversight and a government appointed board of trustees who publish annual accounts. What’s released to the public is not as detailed as I’d like but you can see the areas where the money goes.
The main bulk is to staffing wages and pensions, and buildings upkeep. Travel expenses for royal engagements come out of this too.
For anyone doing Royal ‘duties’ (apart from the male heir) their ‘wage’ comes from the monarch’s Duchy of Lancaster budget. So they are dependent on the monarch for their income. If the heir is male then they get the Duchy of Cornwall income to fund their household.
So when the Queen was alive she was funding all her children apart from Charles. He was funding himself and the households of William and Harry.
The Queen bought Gatcombe for Anne which she lives on, runs as a business and houses both her kids and their families.
Edward and Sophie tried making it in the commercial world away from Royal duties but when that failed plus the scandal of the fake sheik story they were quietly integrated back into the family firm and presumably funded by the Queen and now Charles.
The issue is that if you are going to represent the Crown as a working Royal you have to live within your means as determined by whatever the monarch decides to fund you. When you can’t do that you end up with the shady HIHO York wheeling and dealings.