John purchases a baseball card for $10 that turns out to be so rare that a collector offers to buy it from him for $2,000. Instead, John decides to give the card to his sister (an avid baseball-card collector) as a birthday present. The opportunity cost of John's generosity is $10, the purchase price. $0, because at the time the decision is made, $10 are sunk cost, i.e., at that point there is no cost to John of giving the card away. $2,000, the amount offered by the collector. O $1,005, the average of $10 and $2,000