Well, I can't give any exact amount, but I can say that I bought my 2003 S late last November for a smoking deal! In the last 2 weeks before I bought it, the seller came down $3,000 below Blue Book. This was from Idaho, so yes, snowy country.
I would think the difference is substantial. Have listed my 2001 Box base for sale since September and have only had two tirekickers look at it. Not willing to sell at low price and will wait to get better price in the spring. I am driving it through the winter anyways. Not willing to pick up a used Cayman and incur the wrath of my wife by having a third vehicle sit in the driveway. I was at the stealership yesterday and fortunately they did not have a used Cayman or else I might have been tempted to do a deal. They would allow me about $12k for my Box on a high priced 996 or a 2008 Boxster which was also high priced. No deal....
Buy a used Box now and sell in spring seems to be the way to go....