The Crown Estate, do some research and you'll see it is not owned by the Monarchy it is owned by the UK in the name of the Crown, it is NOT the Monarchs personal property, nor does the Monarch have any claim to the profits from it. The Crown Estates OWN website tells you this, find out for yourselves.
.
No, the Queen "owns" the Crown estate, but "in right of the Crown", not personally.
This is analogous to a Limited Company being a legal person, able to own land. The "Crown" is a sort of legal person, and owns the Crown Estates. Currently, as the Queen is the reigning sovereign, the is the owner of the Crown Estates but they are not personal property, and she is not able to dispose of Crown Estate if she feels like it. She does also have very considerable personal property that are hers to dispose of.
When the Queen passes the Crown to someone else, either by succession or abdication, then tne Crown Estates become the property of that person, say Charles for example, again by right of the Crown. It is, if you like, the legal personage of the sovereign that owns the Crown Estates, not the individual that currently fills that role. The current sovereign is more like a trustee, but without any trustee powers.
The Crown Estates are then admjnistered by an independent board set up by an Act of Parliament, and revenues from the Crown Estates go the the Exchequer, in return for which the Crown gets the payment in the Civil List, which itself was replaced by the Sovereign Grant in 2011.
So, if the state stops sending the "taxpayer money" that is the Civil List/Socereign Grant, it is defaulting on it's obligations under the Act that set up the Civil List/Sovereign Grant, and the result might well be that both the property itself of the Crown Estate, currently valued at some £11.5bn, reverts to the ownership and control of the sovereign, as would the revenue generated from it.
The "UK" does not own the Crown Estates. The institution of the monarchy, currently in the person of the Queen, does. The income goes to the state, and the quid pro quo is the Civil List, and now the Sovereign Grant, which goes to the Queen.
To put that in context, the last published figures show £42.8m of Sovereign Grant, which is what the taxpayer gives the Queen.
On the other hand, the net contribution from the Crown Estates to the Treasury was £285.1m (2015 accounts).
So she gives us £285m and we give her £42m back, leaving the taxpayer with something around £200m-250m profit on the deal for each of the last few years, and some £2bn profit over the decade. The Sovereign Grant is actually set, with the addition of a ceiling and floor, at a rate of 15% of the net revenues ftom the Crown Estates.
As for security costs, it's something of a moot point because we'd have those whatever the form of our Head of State, be it monarch, president or whatever.