An 8 x 12 basic wood shed is going to weigh a good 1500 pounds. Now a metal shed will be lighter by at least half the weight, I would sctually say 1/4 of the weight due to the fact that you don't need near as much material.
How much does an 8x12 Metal storage shed weigh? it will be lighter than an 8x12 wood shed. Your basic 8x12 metal shed will weigh 500-700 lbs. METAL SHED WEIGHT=500-700 LBS.