About a year ago I corresponded with Mercian Bicycles about a custom frame, with weight as well as fit in mind.

They told me that materials did not determine weight so much as they determined ride quality.
They told me to pick the type of tubing based on how it would change the ride and performance, and not on how much it would weigh, because, in the end, most frames, regardless of material, weigh about the same.

Further, they said if I didn't know in what ways the various types and mixes of tubing effected ride and performance, to tell them what kind of riding I do and they would choose the tubing for me.
They suggested that for me, Reynolds 853 tubing would not save that much weight and Reynolds 725 or Reynolds 631 (or a mix of the two) would weigh about the same and give me a better ride, for the kind of riding I do.

A custom Mercian Super Vigorelli frameset delivered to my front door would run about $1100 in US dollars.

http://www.merciancycles.com/frame_vigorelli.asp