There is no formula but I would say 1/2 the price of a new TV of the same size. The price you paid isn't relevant because of continually cheaper TVs. If the TV doesn't have HDMI then it might go for 1/4 or less.
The only possible exceptions would be the Pioneer Kuro Elite and the last generation of Panasonic plasma TVs. These would go for maybe 1/2 or more of their original price.