Personally I think it is too high based on the various SC Targa's that are for sale and have been for a while. The link below is a beautiful Targa that has not found a buyer at $30k and didn't yield a sale on Ebay either. I would search the for sale section for a better idea.
http://forums.pelicanparts.com/porsche-cars-sale/914029-1980-911-sc-targa-owned-23-years.html