How much is a Sterling Armaments HR81 air rifle worth?

Sterling of England was purchased by Benjamin air gun co in 1994 (After Crosman purchased Benjamin) The Sterling line was discontinued.

The HR81 came in calibers of .177, 20 and .22. if fired at a speed of 700/600 FPS. Unfortunately you did not state the current condition of the rifle so I can not give you an estimate of value.