@James Rodgers and @Account ClosedMulti-family unites. They are dissimilar hence would not be comparable. Multifamily residences are utilized widely as income properties and so the function of the property is different too. Appraisal firms are heavily regulated nation wide (FIRREA and Title XI just to name a few - lol and so the primary practices are the same across the board in the US.
The good news is that for a whole lot less money, an appraiser can also give you what we call a desk review which means we look up the pertinent information and comparables from our desks and give you a value ballpark. This will of course come with all kinds disclaimers because it is not a "Complete" appraisal as the appraiser never inspected the property or it's comparables. Typically if a full appraisal cost $400 a desk review may only cost you $150 but since it's a multifamily and like you said there are no comps, it may be higher to include the fact that appraisal fees fluctuate from area to area.
Additionally caution, not all appraisers are worth their salt and not all appraisers are familiar with MFR's so ask around to find the ones with the most experience and just ask them. If you let them know that you are an investor and may need their help from time to time they will probably want you to keep coming back and develop a good working relationship. It means more business and good word of mouth for them.