How wasteful is a voltage transformer?
The bigger problem is the transformer is going to set you back about $100, so it spectacularly fails to make sense for a $40 humidifier.  

Dollar for dollar, if you're coming to North America, you may be better off just adding a couple 240V circuits to  your house, and running the appliances off 240V straight shot.  North America has both 120V and 240V available.