What: Shares of auction house Sotheby's (BID) fell as much as 14% in trading Thursday morning after an analyst downgrade.

So what: Analysts at Stifel downgraded the stock from buy to hold on weakness in the art market. The Sotheby's Contemporary Evening Sale in London yesterday generated sales of just 69.5 million pounds, down 44% from a year ago. This is further evidence that high-end art spending is set to decline as global markets falter and wealthy collectors keep their money closer to the vest.  

Now what: Management has predicted that art sales would fall, but the drop may be faster than analysts and investors expected. What's intriguing today, after Sotheby's hit a new 52-week low, is the value these shares could present to long-term investors. Shares trade at less than 11 times trailing earnings, which could be a value even if the art market's slump continues.

After today's drop, I think the value looks compelling, even if it takes a few years for the core business to turn around.