Rent "discount" for multi-family units vs. equivalent SFP unit

[Jordan Grimstad]

Say I have a SFP that rents at market rent for $x/month, and I find a multi-family property where the units/amenities are otherwise equivalent to the SFP (same bed/bath, location, finishings, parking spots, etc.). Is there a reliable rule-of-thumb that relates the SFP rent to the likely market rent for the units in the multi-family? 

Phrased another way - is there a reliable way to determine the price of privacy offered by a SFP and decrement this price as privacy decreases?