There is a very interesting video on the Crown Estate's (net contributor to HMRC even after paying the Civil List and subsequently the Sovereign Support Grant) and how the Royalty set it up and what would happen to it if the Government were to ever renege on funding the Royal Family (i.e. the crown estates would revert back to the ownership of the Royal Family). I can't find it just at the moment.
Could the Royals afford to retain the estates if they have to pay taxes instead of receiving tax payer support? The Tatton Estate near me used to belong to one of the Lord Chancellors but finished up being passed to the National Trust. Many other estates have had similar fates.
Then there is the issue of where to house the President (bearing in mind you've just lost all the Royal households) and what savings there maybe compared with having a monarchy (i.e. you would still need to provide them with protection).
Downing Street? If we have a President we won't need a PM and Downing Street already has a security presence and 24/7 policing when the PM is at Downing St.
Then, even if the UK got rid of the Royal Family there are a lot of places that they would still be head of state for and so they would still need protecting whilst they were using any of their properties in the UK (much in the same way as the police have to provide a certain level of protection to diplomats).
If they were to remain head of state for other countries wouldn't said other countries be inviting Royals to come and live in the countries they remain head of state for?