If something is unaffordable, it's too expensive. A house that costs millions of dollars is unaffordable for almost everyone.
This adjective often describes things that cost too much money for the average person, like unaffordable rents in a particular city or unaffordable health care. When you can afford something, it's within financial reach; you can manage to purchase it. Unaffordable things are priced too high for your budget or bank account, like those ultra-rare sneakers you covet.