Priroities. Covert Affairs HAS to go to other countries, or the show doesn't work. So they work the budget appropriately. Actors sign on for it, several episodes or even a season is there, etc.
For Castle, it's a novelty gimmick. Would cost a lot to move the whole production there for just part of an episode or so, so easier to just fake it.
For Castle, it's a novelty gimmick. Would cost a lot to move the whole production there for just part of an episode or so, so easier to just fake it.