I don't know what lens, presumably on an SLR, would be worth but, depending on the model of camera and the condition it is in, you might not even get $150 for it on eBay.
I could not say with any authority that the lens is or is not worth $500.
Regardless, the merchandise is not in prime condition. Depending on the shop and the temperament of the owner, you could use this fungus problem as a bargaining plank.
Some antique shops will bargain. Some will not. I have gone into shops and seen Griswold cast iron skillets marked or more times their value. When I asked the owner, he basically said, "That's the price." I went down the street and bought the exact same type of Griswold skillet in better condition for half the price. On other occasions, when I went into the shop and asked the price, the guy basically said, "Make me an offer."
(P.S. I live in the town where Griswold cast iron skillets were once made.)
Bottom line: Don't be afraid to ask for a bargain and don't be afraid to walk out.