Any player gets residence for tax purposes, which always a sore point with the rest of the French leagues but Monaco always finds a way around the objections or just ignores them. He could buy a house in Monaco; that is an expensive hobby because a flat over the border might be 400, 000 euro but the identical flat inside might be well over a million or more; but he'll be allowed to have residence if he only takes a lease. There is no income tax but the rates are quite high, largely for the provision of water.
Monaco have got themselves a billionaire owner who somewhat resembles the early Abramovich, wanting big names, often past their best. Ronaldo would be an exception. For him, 85 million is nothing. If this goes on , I might have to get another season ticket! The team went from poor to dreadful and has only just got back into the first division. Their ground only holds 15,000 and I have never seen it full, even in their best times. But the admission prices, and the seasons, are so cheap that they'd make English fans weep. The service is excellent, with ushers to show you to your seat but, gratifyingly, the food in the ground is just as bad as in English grounds.