I don’t think there is an exact pound-for-pound value because so much of what they provide isn’t quantifiable - soft power, influence, cultural power, constitutional stability, historical preservation, a link to our past and our future, etc. Those things have a value or generate value but it’s hard to say exactly how much.
The thing is, countries that abolish the monarchy still have to do all of those things somehow. So the idea that if we abolished the monarchy we’d suddenly have a spare £X million to spend on Our Glorious NHS is for the birds.
If you look at France, they don’t have spare cash lying around, they haven’t done away with pomp and ceremony, they don’t not bother with trying to generate soft power and cultural power, they just do it in different ways (which also cost public investment).
I also find the UK republicans’ vision of a country with all the supposed “unnecessaries” stripped away - no pomp & circumstance, no public events which require policing, nobody who becomes internationally important enough to require protection, art and antiquities sold off, no public money spent preserving old buildings and parks, no “nice things” - all a bit depressing.