I'm not sure I follow exactly what you mean by "fabricated structural steel"...
However, I looked up our recent (past month) cost for the following two items:
Wide flange, 4" X 13 lb/ft
Channel, 3" X 4.1 lb/ft
In both cases the cost was about 60 cents per pound. Obviously, these are the smallest common sizes of both wide flange beam and channel, so it might vary a little based on shape and size. Two items is nothing to live and die by for sure. However, for a rough approximation, without more info, that's what I can give you.
If by "fabricated" you mean steel that has been machined, that would depend greatly on the design. A 3 pound piece with 10 hours of machining required will be way more expensive than a 40 pound beam with a few drilled holes.
MechEng2005