I did a little weight test of some parts and here are some of the results. Bear in mind that my scale is not to the ntthhh degree accurate.
My stock '73 dash gutted, meaning without any gauges, wires, vents was roughly = 20.5 lbs
My '90 240sx dash With vents, but (Without gauges, wires) = 12 lbs
Aftermarket:
Reaction Research "Subtle z" rear quarter panels (which bond over the cut, stock metal panels) weigh in at roughly 5 lbs.
Hybridz vendor (Rayven14) fiberglass interior door panels (With my added tweeter hole and built up material) = 2.5 lbs
I'm sure there is some fluctuation to my scale but this is what I came up with, and hope it helps some.