They've been told that they've got to pay commercial rent on Frogmore, now that they're no longer working royals. That's going to be a lot of money for somewhere they're only using for a few weeks a year, but I don't know what the alternative is - they can hardly put an ad in the local paper offering it for rent, much as I'm sure loads of people would love the idea! What a waste of a multi-million pound refurbishment.
It's not really practical to spend half the year in one country and half in another, as was originally being suggested. I know some people do, but that's usually retired people trying to avoid bad weather. You can't be running backwards and forwards with a toddler. Once Archie's old enough for school, their travel options are going to be very limited.