When a really iconic piece by a popular artist on his signature character makes its way into the marketplace, you sometimes end up with two or three wealthy collectors who really want to add that piece to his collection, then you get the fierce bidding that drives up the sale price.  If you're a huge Dark Knight fan--and a lot of people are, given the book's influence, popularity, and the fact that it's never been out of print since its initial publication--there are a handful of images from that series that immediately come to mind when you think of that book.

If I had more money than I knew what to do with, I could see myself bidding on those, or on the Amazing Spider-Man #328 cover.  McFarlane was one of my favorite artists in the late 1980s/early 1990s, I loved his Spider-Man, I loved his Hulk, I loved the Acts of Vengeance crossover, and I probably read that issue twenty times the year it came out. Buying that, or the covers to Amazing Spider-Man #300, 301, or 312...I'd do that in a second if I had an essentially bottomless bank account.  Just like I'd do for favorite covers by Frank Miller, JB, or dozens of other favorite artists.