Superlight Steel?
In general, I think that bike weight, and frame weight in particular, are not all that important. That said, I occasionally enjoy playing the weight weenie game. I built up a lugged, steel Heron Road frame into a 17.3-pound bike with a carbon fork and other lightweight goodies. Had I swapped in the Zipp wheels I got later, it would have been well under 17 pounds. The bare frame was 4.25 pounds. Complete retail price of this concoction was about $4000 or what a 17-pound Specialized or Trek was going for then (2 years ago).
Think of it this way, take the components of a 15-pound bike off of its 2.25-pound frame and put them on my Heron. 17 pounds.
Just for fun, I recently weighed a 1971 Raleigh International I just bought. Even with the heavy Brooks saddle, Weinmann center-pull brakes, and lots of chrome on its 531 frame, it came in at 22.5 pounds.