I use a rule of thumb that the gross rent should be 1%. So a duplex that is renting for $1500.00 a month should be $150,000. Of course deferred maintainence or very good condition could alter this one way or the other.... more
Unless you are going to live in one side, you need to be asking about the rents. A capitalization of about 10% should guide you to what it is worth once you know the rents.
Let's say you get $700/month each side. Multiple by 12 months to get your annual and then divide by 10%. I show a list of about $168,000 would be fair market as an investment. If you can grab it for less then congrats! You win!