See: https://www.gov.uk/government/publications/sovereign-grant-a...
"In exchange for this public support, The Queen surrenders the revenue from The Crown Estate to the government. Over the last ten years, the revenue paid to the Exchequer is £3 billion for public spending."
Basically by allowing the monarchy a fund to be used to carry out their public (and private) duties, the surplus of royal holdings revenue goes into public spending. I don't know the exact numbers but the situation is a LOT more complex than "the taxpayer pays for the monarchs, reeee".
See: https://www.bbc.com/news/explainers-57559653
and
Here are some of the key figures from the royal accounts for 2020-2021:
https://www.bloomberg.com/news/articles/2022-06-29/how-much-...
The RF owns the Crown Estate. Of this estate the UK government sits on/uses part of that estate. Therefore they're essentially paying "rent" for what they're using.
However it used to be that the RF received all of the income from the Crown Estate. The SG was an agreement that they would forgo taking all of the income and only a portion of it; the rest goes to public spending.
Hell, they even had to invoke a provision to make up the amount should it fall: "A decrease in the Crown Estate's rental income during the COVID-19 pandemic led to the first use of the provision that prevents the value of the Sovereign Grant from falling, with the Treasury committing to make up the shortfall.".
It's a weird system for sure. The treasury pays out rent for all Royal Holdings that are being used by the goverment/public. Then 15%~ of that rent (plus income from other parts of the holdings) is available to the RF as a part of the SG. The rest goes back to the public.
There is a valid point made that security isn't included in these costs/returns, though. But at the end of the day, it's because the RF owns huge swathes of land and businesses, etc that they are paid. Sure they could be stripped of them but that sets a precedent.
Did you miss the whole Platinum Jubilee celebration two months ago for the Queen? [1] The scope of that was also over the top and costs 10's of millions of pounds. There was also the Saphire Jubilee 5 years ago, the Diamond Jubilee in 2012, the Golden Jubilee in 2002, there's at least a few others as well.
What are the Crown Estate assets worth? How many billions? And the public are not only expected to foot the bill for this over the top pageantry but they are supposed to do so with unwavering fealty? And at a time when the average person is worried about skyrocketing inflation and energy costs?
[1] https://en.wikipedia.org/wiki/Platinum_Jubilee_of_Elizabeth_...