depending on the condition and age, the typical price is $300-$400 and purhaps $500 for a brand new one from someone who just bought an ST but didn't want or need the tonneau cover. That also depends upon if you are willing to ship the tornneau cover, have the buyer pick it up, or meet the buyer half way to make the exchange. They are a little more difficult to sell at the higher prices if the buyer has to pay a heavy shipping charge so most people prefer to sell to someone within driving distance.
...Rich