I have been asked to reply on behalf of the Treasury.
HM Treasury pays Sovereign Grant to the Royal Household, out of which maintenance costs are met, in line with the Royal Household's priorities. Before the Sovereign Grant was introduced in 2012, the Department for Culture, Media and Sport provided the Royal Household with an annual grant for property maintenance. It was the responsibility of the Royal Household to decide how these funds were allocated.
Details of the overall spending allocated to maintenance of Royal properties are available in the Royal Finance Reports on the Monarchy website. The most recent Sovereign Grant Report records that £300,000 was spent on the refurbishment of the Mausoleum at Frogmore in 2012-13.