I don't know if you mean this painting specifically, or paintings in general, but obviously, art is worth what someone is willing to pay for it. Now, there are many reasons a person might pay a lot for a piece, or how a piece might end up selling for more than an expert would expect. Here are just a handful:

1. Someone buys expensive art and donates it to a museum for a nice tax deduction. The more the painting is, the more the deduction. The person who bought Number 17A immediately donated it to a museum.

2. Two collectors are both really rich and both really interested in a piece and get into a bidding war. It only takes two, but sometimes there are many people who want a particular piece or a piece from a particular artist.

3. A collector (or dealer) might try to spur interest in an artist by overpaying for a piece to try and reset the price for said artist.

4. You can launder money by buying or selling art at auction.