People tend to ask for a finished basement as though it were one product. It is closer to six, and which one you are buying changes the price far more than the floor area does.
A living room is the cheapest thing to put down there, because it asks nothing of the plumbing and very little of the code. A bedroom costs more than its size suggests, because it needs a compliant escape opening and that usually means cutting concrete. A bathroom is governed almost entirely by where the main drain already sits. A theater is a geometry problem before it is a finishing problem, and an office lives or dies on circuits, daylight and the ability to shut a door.
The pages below describe each one from that angle: what it involves, what carries the cost, and what to decide before anyone starts.