I don’t know that it’s that cut and dried. For one, the cottage was supposedly a gift.
Was paying rent ever laid out as a stipulation of that gift? And if they fronted their own money for fixtures and other design elements within the home then they’ve theoretically improved the value of that house above what it would have been without their investment. Was this a gift from employer to employee contingent on continued employment? Or was this a gift from a grandmother to her grandson completely separate from their working relationship? Does Philip get to keep living in royal housing post-retirement because that’s a perk of his retirement benefits, or because he’s the Queen’s husband? Will William and Kate be billed for renovations or asked to pay back-rent should they decide to move out of one of their royal residences? (I believe they have two.)
Philip was given the title of Prince in 1957 and retired from royal duties in 2017 = 50 years.
Harry has held the title of Prince for 35 years.
Harry should get 70% of whatever Philip’s retirement package was. Problem solved.