Here are a couple of results for the fronts from the large auction site.
Porsche 930 Steel Turbo Front Fender Flares
*NOS* Porsche 76-89 930 OEM Front 9" Turbo Flares
Not sure how much more the rears would be worth these days, or how much more they are worth compared to the fronts. In the end you need all four, but rears could certainly be used without fronts, but fronts without rears would look rather silly. I'd say add at least 25% for the rears.