Are Artworks Valued Like Stocks?

I never thought about the economics of music performances and performers because I am one of those who have never got much past Mozart. But I do have a question about the Art World.

As the risk of being a philistine, I would like to say I cannot see how artworks are evaluated in monetary terms. The notion that an artist like Picasso could doodle on a paper napkin and have it be worth hundreds of thousands of dollars, if not even more, makes me wonder if Art is not totally evaluated like stocks: not on the basis of intrinsic value but as a way of making money in a down or up market without any concern for intrinsic value. What makes my neighbor's painting worth $300 at the local art center and something by a long-dead Fleming worth $3,000,000? I confess I cannot say, because I think value is entirely in the mind and eye of the beholder and in the pocketbook of the investor -- with no other meaning. I would love some comments about this. I am not afraid of being considered an art philistine because I have a healthy respect for my own judgment in many, not all, things. Unlike Monsieur Jourdain, I know I am writing prose and don't feel excited by the discovery!