Four hidden cards (60, -40, -20, 0), one in the opponent's pocket: walk away, take the pocket card, or pay $6 to peek at a table card. What is the optimal strategy?

Four cards worth 60, -40, -20 and 0 lie face down. The opponent picks one at random and keeps it in his pocket. Each round you may (1) walk away and gain 0, (2) take the pocket card and gain its value, or (3) pay $6 to peek at one of the cards on the table, after which you can no longer take that table card. Is there an optimal strategy?