As for the great historical works, eh, you'll find works from the big name artists throughout history are worth a ton due simply to the name they're attached to. You see this a lot with guys like Rembrandt. A painting will sell for $30,000 only to later be shown to be a fraud, making it worthless. If the painting itself was great, the artist wouldn't matter.
This, of course, is also true. And partly goes in with what I said above about "radical" artists. If the right person (Tracey Emin, Damien Hirst) craps in a bucket, its worth a cool ten grand, but if one of us did it, it would be a bucket full of feces. See also the painting "Sir Nicolas Serota makes an Acquisitions Decision."
There are some art students/lovers/connoisseurs/whatevers who say the big point of modern art is that these artists are actually the greatest geniuses of all time for convincing people that they should receive millions of dollars for crapping in buckets. And naturally, anyone who feels this way is part of the problem.