Tech,
You are right softie's tend to have their own marketplace which is converse to all other targas. They sell for a healthy premium over a coupe. There is a rough 68S coupe (Budge's) currently available here on Pelican for $30,000ish. Here is a beautiful ebay coupe that went recently for $50,000.
Porsche : 911 - eBay (item 220724565953 end time Jan-23-11 04:25:40 PST)
Yet
driver SWB S Targas command a stronger price. Here is one that sold here last year for $55,000.......
http://forums.pelicanparts.com/porsche-cars-sale/531668-fs-1968-911s-swt.html
They have sold into the $150,000 range for "original" cars with documentation.
What did The Green (x gallagly) S softie w/ the non-matching but correct S motor sell for? It was beautifuly restored and California Restoration wanted $89,500. They currently have a white car which I believe they wanted $110,000ish when I saw it last year at the Lit show swap meet.
Cars - For Sale - Porsche 911 - 1967 Porsche 911S SWT - Light Ivory - California Porsche Restoration
These cars have enough room in them to have a "Master" shop restore and get out with a little extra. The Hopkins/Cabel restored car sold for $121,000 in 2007.
IMO, a 68S softie, unless parts are missing like motor, trans etc.. will bring a good price. I recommend to sell as is and let the eventual owner complete the restoration. A number of years ago one that was nicely optioned rolling rust sold in the $30,000 range on EBay.
Here is another thread that might help;
FS: 1967 Porsche 911 S Soft Window Targa Bahama Yellow 64k Orig. Miles