I believe the "values" have bottomed out, yet prices will vary based on the condition of the individual car. For instance if the car has been maintained to Porsche specs: Oil change every 15,000 miles or annually with Mobil 1 0-40 is not nearly as good as every 5,000 miles with a better 5-40 oil.
I'm guessing the sub $20K 996's you see are 1999 cars which tend to have the most problems. There is a good chance buying a $18K 996 will be more expensive car to own for 2 yrs than a 996 bought for $22K. There is also a good chance once you own a 996 you will keep it until you are unable to physically get in & out of it under your own power.
Read all you can on this & other forums & don't be penny wise & pound foolish about obtaining this dream machine!