Duplex and/or multifamily pricing

[Eric Siebert]

I'm curious as to how you go about judging the price of a duplex and/or small multifamily.  I'm looking at a property that has an on market price of around 230k. The Zillow estimate( I know, I know), says it's worth 170K and the assessor says 120K. With a single family home it is so much easier to judge the price by looking at comps in the neighborhood. What is the best way to go about plugging numbers into a formula that says "go/no go?

Thanks