In the most detailed review ever of royal property arrangements, the National Audit Office (NAO) report on Friday showed some leases were based on commercial valuations, while for others, senior figures paid no rent or negligible amounts for their properties.
The watchdog carried out its analysis after UK parliament's Public Accounts Committee said last December it would hold an inquiry into the issue amid questions about the lease Mountbatten-Windsor held for the Royal Lodge mansion on the King's Windsor estate.
Charles has since forced his brother to move out of his home and stripped him of all his titles over his ties to the late US sex offender Jeffrey Epstein. The issue has been seized on by critics of the monarchy, who increasingly question their wealth.
"We hope that the findings will help correct, clarify or contextualise a number of points regarding royal properties," a Buckingham Palace spokesperson said.
The NAO said it had examined the property agreements the family had with the Royal Household and with the Crown Estate, a vast property portfolio belonging to the monarchy but which is independently run with all its profits going to the Treasury.
The spending watchdog offered no opinion on whether the deals represented good value for money for taxpayers.
Under a deal struck in 2003, Mountbatten-Windsor obtained a 75-year lease for Royal Lodge in return for a STG1 million ($A1.9 million) up front payment and a commitment to perform STG7.5 million in renovations, which he duly carried out.
Thereafter, he paid a "peppercorn rent" - effectively nothing - for the mansion and the eight cottages on its 40-hectare estate. He sublet and kept the rent for three of the cottages, which only became unoccupied in April.
Neither the NAO nor the Crown Estate, an independent commercial business, had details on that income.
Mountbatten-Windsor paid STG12,922 a year for another property, Sunninghill Park in Windsor, which was used by a member of his staff. That lease will end in July next year.
A spokesperson for The Crown Estate said leases with members of the royal family were "in line with independent, professional advice and open market valuations".
The NAO report also showed the Royal Household provided seven official residences at Kensington Palace and St James's Palace at no charge for royals who carried out public duties, such as heir Prince William and the king's sister Princess Anne, in a long-standing deal in return for their official work.
Three 'non-working' royals - Mountbatten-Windsor's daughters, Princesses Eugenie and Beatrice, and the Duke of Kent, the king's first cousin once removed - paid a rent set at 60 per cent of the open market to reflect the fact only a limited number of people would be able to live in such premises for security reasons.
However, the NAO found the king was paying their rents out of the Privy Purse, the sovereign's private finances, and that the rents set had not always matched the 60 per cent valuation.
