Offer your thoughts or opinion on the input financial query or topic using your financial background.

What's an economic explanation for why greeting cards are so expensive? 
Competition, or actually lack of competition, mostly due to a demand curve that has minimal change due to price. You would buy the equivalent, cheaper option if it was available, but the store has little interest in offering multiple, competing options that would drive their same store revenue down. And the competing stores (Grocery, Department, Drug, Card) have similar overhead costs (floor space, lights, personnel). Most carry the cards for incremental revenue, and observe little advantage to lower price for a card (customers seldom buy more cards due to a lower price). Thus they mark the price to what (most) customers are willing to pay. You may choose to shop the various stores and find the one that has a (slightly) better pricing for cards, and then stop at that store when you want to buy a card. But many cards are sold as an incremental purchase as part of a larger shopping trip (convenience), as the customer combines trips (reduce the time spent shopping, albeit not reducing the money spent).