Call around to your local stone & granite suppliers and see what they price theirs at.
The price will typically depend on the quality of the granite (i.e.- color, veins, pattern, defects, chips, pits, scratches, etc.).
"Whatever the market will bear." Not a smart-aleck or facetious answer, but simply the realistic truth. If it's pristine and the right color, putting it on CL with an "open to offers" asking price should generate some interest.