I am not privy to the private accounts of either of these estates or of King Charles’ private income since his Mother died, so I have no idea if he can afford to maintain both these estates or not.
Balmoral receives a good income from salmon fishing rights and other rents but I have read that Charles was considering opening Balmoral to the public when it is not in use, which would add to the estate income.
I have seen suggestions that the King may turn Balmoral into a permanent museum and use one of his other, smaller Scottish homes, such as Birkhall, which he has often used when visiting Scotland.
I presume Sandringham has an agricultural estate which earns money to offset the maintenance costs of the main house too.
All I know about the finances of Sandringham is that the last major renovations a few years ago, (half a million pounds worth) was paid for by the Queen from her private income.