Why are the royalists the only ones who understand the monarchies finances.?

They are very certain aren't they. I wonder if they were surprised when the Queen was reported as avoiding paying tax, something she had apparently volunteered for.

Because they are only ones who take the trouble to find out the facts, and because they don't have the same bias that republicans have. Fictions like "the queen avoids paying tax" are absurd and deliberately misinterpret the situation.

If the Queen holds monies in Bermuda or the Cayman Islands, territories for which she is the Head of State, that is a matter for the people administering the estate and tax is voluntarily paid on the proceeds from the transactions that she receives. The Duchy is audited.

I am a royalist and am aware that the Royal Family bring in 100s of millions £ every year. This money comes from people who buy from companies with Royal warrants and the "I want to look like Kate and/or Meghan" brigade, copying clothes, shoes, hats and handbags. This is offset by the £348,000,000 the taxpayer forks out for travel and security. This system isn't perfect but, far better than a republic.

It's not rocket science. All the information is on the internet. However, republicans like to hang onto the myth that the royal family costs the taxpayer money. The Exchequer gets 100% of the profit from the Crown Estate and 25% of the profits from the Royal Duchies. This is all personal wealth of the monarch who would be far better off paying Corporation Tax like the rest of us. Out of the money we get from them, we pay the costs of royalty - and we make around £500 million a year out of the deal. So any republicans planning to be President had better be in a position to fork out a similar amount of money for the privilege.

BTW I'm not a royalist. If someone comes up with a better system, I'd support it. All that's on the table is an expensive regime with a politician in charge. If you want to avoid President Boris (aka Trump II), stick with Betty and her heirs.