3.16 lb *0.45359 kg/lb * 1000 g/kg / 2.7 g/cm3 = 530.87 cm3
The "strong & flexible plastic" is $1.40/cm3, so $743
. Sandstone is $0.75, so half that.
I am a little confused / disappointed because I thought I read a post about getting an ErgoDox case printed for $35.
Maybe Shapeways could make a scale model.
But to my materials question, it seems that the densities of brass, bronze, and steel are within about 10% of each other, and about 3x aluminum, so a full keyboard+numpad case would go from ~4 lb to ~12 lb.