Well, that sounds like a simple window mounted unit, when they probably meant a whole central air (ducts through the whole house) kind of system.
Yeah, but a window mounted unit is good enough. Keeps your whole place cool. It sounds like everybody has their windows open anyway, and this way you can store them or sell them when you don't need it anymore.
A lot of landlords & HOAs disallow window units. They also can compromise your home security
So dystopian. "No you can't put a magic cooling box in your window even though it will solve all your problems, because I say so."
Well, it depends on how it's installed, the power, the model etc. Built-in central air models, especially combined with heating, can run pretty pricey. If it's a big house and it's a new model, with all regulation-required energy-saving features, it's a five-figure territory. Of course, if you need to cool only one room, you can get small second-hand one for much cheaper.