Proper way to compare multifamily (multiplex) home values in an area

[Devin Beverage]

When determining whether a multifamily property is over or under-priced, how should comps be considered? 

Ideally I would image we would try to compare them to other multi-family homes for sale in the area, but if there are none and we have to go by single family home values, how do we evaluate whether or not the multifamily unit is a good deal or not? 
For example, for a 2 bed 1 bath (each) duplex, would we divide the sale price by 2 and compare to 2 bed, 1 bath homes in the area? Or would we compare the whole sale price to 4 bed, 2 bath homes in the area?