Tim, These smaller multi family deals (20 units or less) aren't always valued based on cap rates. As Andrew stated value will also be based off comparable properties. A commercial appraiser is likely to use both methods and average them. If you are trying to figure out a "fair" price, I think you also have to consider not just the money, but the time you'll take in taking care of the deferred maintenance as well as the time it will take to raise rents to market especially if it's in a state that only allows you to raise rent by a small percentage. Also is the convenience factor, if you are paying cash or have 'easy' financing that's more than a full bank loan, the seller should factor that into the price especially if you can't get conventional financing due to their poor book keeping or management. This is a great question because there is such complexity to the answer. It's not just cap rate, comps, repairs, seller motivation alone that go into factoring the price, but all of them, and, to what extent is the weight of each of those categories...again depends on the situation.
Be prepared though if this is one of your first MF deals. Appraisals are often $1,500-$3,000. Also make sure you get an environmental report, even a desktop one can save you hundreds of thousands of dollars down the road. Financing on these whether again short (private money) or long (institutional money) will be more expensive and require a larger down payment both because the nature of these deals are riskier than SFR's but also, as you stated, this is your first in this asset class.
Glad to see you moving into that natural progression from SFR to multifamily.