I am aware of the fact that his exwife Sarah is living there, but still, do they need a 30 room residence? And even as their children are visiting there, that is no reason IMO. Eugenie and Beatrice have homes of their own. Even the heir, William is not living in such a large house, and he has a wife and three children and they don't want to move into Royal Lodge. If you compare the different houses the children of the late Queen are living in, there are enormous differences. Anne's Gatcombe Park was given to her by the Queen (correct me if I am wrong). As far as I know both of her children are living on the compound with their families. You never hear of necessary repairs of the buildings. Edward and Sophie are living in Bagshot, which is huge, but parts of the mansion are rented out for offices, and there are only three of them residing there.
Want I wanted to say, is that Andrew's privilege of living at Royal Lodge with his exwife and nobody else seems to be huge in comparison to his siblings. Maybe that is the reason why Charles is so keen about getting him out and moving into a smaller place like Frogmore Cottage, which after the restauration probably is in much better condition.
The question is, does King Charles have to save money? They are so very rich, maybe there are other reasons for Charles' decisions, there might antagonisms going far back into their childhood, Andrew alsways being the one who was pampered by their mother and even their father, who knows?