“How much do you suppose the US president costs? Or the French president?”
Irrelevant as the Queen has no “real” function, she is only a symbol.
She is the equivalent of Marianne. And Marianne cost not a penny…
How much for the Prime Minister, on the top of the Queen? This has to be included in the pricing...

French and USA President have to power to go to war and to push THE button…
Then the expenses stop at the President and wife. It doesn’t extend to sons, daughters, cousins and so on.
You don’t have to protect the President’s heir, as you don’t know who it will be…