I know all commissions are negotiable, but what's a typical commission for multi-family buildings in 2017?

What kind of fee are your referring to? Lease or sale? Who is the listing broker or are you representing a qualified buyer? Is this a 1031 exchange transaction. What is the listed price of the building?
Broker fees will typically fall in the 3-4-5-6% range depending on a number of variables

6%

good

reasonable