6 Units (3 duplexes) ... help w/ value with the income approach

[John Ringgold]

I have happened upon what seems like a nice opportunity with 6 doors. As a Realtor, I can typically find comparable sales, but in this case I can't find any.

I chatted with an appraiser friend who said because it is 6 units, the value should be determined via the income approach.  I have looked online and haven't found a good online calculator or a simple to understand formula for finding value this way.  Not something I typically need to do.

Any good resources or thoughts to help me find the value to ensure my rental calculations work before making the decision if we should make an offer.