While it's impossible to state an "average" difference in weight, your higher grades of steel-Reynolds 853 for example-will weigh less than chromoly. As 853 is stronger the tubes can be made thinner, leading to a decent difference in weight. I've seen it stated where the tubes on high quality steel could be made up to 1/2 the thickness of chromoly and still have the strength needed. That would be a significant weight diff.