Why do some of us get so cranky when some crummy photograph sells for millions of dollars?
Consider the diamond. This is a more of less common rock (not common everywhere, but where they are found there are lots and lots and lots of them) which is easily synthesized. For some reason, though, diamonds are very expensive. Why?
For social reasons, we as a society want them to be expensive. The social reasons are carefully nurtured by DeBeers but belong essentially to us. DeBeers and society as a whole conspire to keep the supply of diamonds artificially restricted, so that they remain expensive. The diamonds we own remain valuable in some sense, and we enjoy the romance of the diamond as thing. Everybody wins. Well, except the guys digging the diamonds out of the ground.
In the same way, we as a society and those members of our society who are very rich, want there to exist very very expensive pieces of art. The rich want to possess things of rarity and value, and the rest of us want to cluck our tongues disapprovingly or to enjoy fantasies of being rich. The trouble with art is much the same as the trouble with diamonds -- there is just so damn much of it. How on earth can art be valuable or, more to the point, expensive, when it is being produced by the trainload on a more of less daily basis all across the world? Art isn't as common as mud, but it's pretty common.
You do it the same way you make diamonds valuable. You invent gatekeepers and tastemakers to artificially restrict the supply of Art, or at any rate Fine Art. These gatekeepers are primary market collectors, curators, and so on. Their job is similar to that of a fashion designer, they must have an intuitive grasp of what the market is ready for. They must discover new artists and new art at the right rate, to keep the market supplied while keeping prices up. The art must be novel, but not too novel (markets are always ready for something new, but never for something too new). Ideally, it would also be good -- whatever that means. Surely the gatekeepers strive to find good art, of course they do. Still, when a million bucks is on the line we are justified in speculating that closing a deal might take priority over good.
New art must of course be justified as good. Nobody wants to buy a piece that's bad. Happily, the art world comes free with a more or less infinite supply of technical jargon and meaningless phrases which can be used to justify practically anything as good. To reiterate, any piece might well be good, but the point is that it needs to be seen as good. Always assuming that previous sentence means anything at all.
And so we have things like Gursky's Rhine II, which might be good, might be bad, I don't have an opinion. Any competent technician could have made it, and the concept doesn't strike me as particularly strong. It's printed really really big, though. Does it evoke something? Certainly not any more, now all it evokes is HOLY CRAP FOUR MILLION DOLLARS?!! The Fine Art world has successfully destroyed this as a piece of actual art.
We have Cindy Sherman's Untitled #96 which, like all of Cindy Sherman's work, appears to be the work of a not very bright college sophomore (this is at least in part because not very bright college sophomores tend to copy Sherman and her derivatives). This one is a trifle under four million dollars.
It doesn't matter, though. Gursky and Sherman are valuable because they are expensive, and expensive because they are valuable. Whether they are good or bad, or what those words even mean, is entirely irrelevant. The Fine Art world demands it, we demand it as a society, and the gatekeepers and tastemakers can only do their best at supplying what is demanded. Everyone makes a tidy profit, everyone's happy.
It's not fair that artists are selected essentially at random to win the Fine Art lottery, but what can you do? That's how it is.