I intended to object, but apparently the royals cost the government ~500MM GBP/year. Which is a lot, but less than 5% of their tourism industry, so it seems possible.
How much of that budget is maintenance of very very old buildings?
They definitely do, but a lot of this admittedly is to do with the ability they have to raise revenue on things they privately own; only a handful of Royals are on what is called the Civil List (direct payment from government for their jobs, which are not inconsiderable.)
Tourists would also come to see their remains and preserved trappings, as they do in every other country with a former monarchy.