Driver condition stock SC in that shape is 28-32K all day. That one being modified narrows your audience to someone looking for a toy, not an investment. There are a few wide conversions to choose from:
http://forums.pelicanparts.com/porsche-cars-sale/876063-1977-911-wide-body.html
This one is really cool and great interior.... granted, only a 2.7 motor but only got to 21K on ebay.
I'd guess somewhere between 25-30K for yours. Just a guess. Cool car, I love it.