I'm a local GC :). But I've spent 20 years doing single-family residential, not light commercial. That's why I was looking for a ballpark.
@Andrew Taylor It sounds like you are talking about 5+ units, basically a small apartment complex. My experience is in 2-4 unit building, primarily in duplex houses. Based on my experience, a two story duplex costs me 5-10% less than a single story duplex, on a $/sf basis (basically savings in the roofing and foundation, a bit more for framing due to the second story). You will have a much bigger jump in costs once you go over 2 units due to the change in building code (going from IRC to IBC), which brings in all the costly fire and life saving stuff. For me, I see an increase of 25-30% on a $/sf basis when going past 2 units.
It hope it goes without saying that building multifamily 2+ is always going to be more $/sf due to the separate utilities, extra kitchens, etc.