I'm sure there are exceptions to this but I think most junkyards price things based on what they are not what engine they go on... Don't act like it's any big deal, just go get a Volkswagen Corrado valve cover. Unless they know there's something special about them they shouldn't charge you more than they would charge you for say, a Volvo valve cover.
I wouldn't pay more than $20 for one at a junkyard but maybe I'm spoiled, around here people don't think old European cars are anything special. It's the Japanese stuff that commands ridiculous prices.