What do they own themselves? I'm sure it's plenty to keep them in the manner they're accustomed even if we divest them of their other houses.
There's Balmoral (which includes Birkhall, Craigowan Lodge and Tam-na_Ghar) and Sandringham (which includes Anmer Hall).
Gatcombe Park.
Highgrove is owned by the Duchy of Cornwall, so potentially privately owned or not.
Llynwynermod is also owned by the Duchy of Cornwall.
Ray Mill is Camilla's and Old Mill next door was reportedly purchased by Charles.