Since pretty much any kind of bike can be pressed into service as a commuter, weights are going to be all over the place.
Still the classic commuters are portly. I'm thinking about the European three speeds with fully enclosed chain guards. Those are the gold medal standard for commuters in many ways. Three speed hubs are pretty bombproof and seldom need adjustment. The fully enclosed chain guard is a drag when fixing a flat (in fact you'll want to repair the tube on the wheel rather than remove one of those things) but they are great for keeping pants legs clean and keeping the chain clean as well in all kinds of weather. They have generator lights, a super strong rack, and usu. a built in lock. I'll bet those bikes weigh upwards of 40 lbs.
Americans are pretty eclectic as to the kinds of bikes that get pressed into service for commuting. Still if you are looking for a bike that can take a reasonably stout tire, racks, fenders, etc., 30 lbs give or take a few lbs is probably a pretty reasonable weight. I currently have two vintage bikes I've set up as commuters: a 1993 Bridgestone BB-1 and a 1988 Specialized Stumpjumper Comp that I converted to drop bars and bar ends for commuting. The Stumpy has double butted Tange tubing; the Bridgestone is plain gauge Tange tubing. They both weigh the same with leather saddles, stout tires, racks, fenders, etc. at 32 lbs. Both bikes were CL finds and cheap; recycling an old bike as a commuter is a fun project. They both soak up bumps beautifully:
