Also, when a home is lisred in MLS- say its a 3,000 sqft above ground and 1,000 sqft in full finished basement. total 4,000 sqft. in MLS there is a place to check off if the qouted sqft. includes finished basement or not. As was said on an earlier answer, the value a finished basement adds is not the same as above ground. It depends on the level of finish and if there are full size windows or sliders letting in natural light.
Best of luck