I know that percentages are given for duralumin, steel, and electrum. Electrum is mentioned at the begining of HoA, steel is mentioned by kelsier somewhere in FE, and duralumin is mentioned when Vin gets the sample and tests it to see if it's a valid alloy in WoA.
I don't have time to look them up but I think that they are:
Steel: 91% Iron, 9% Carbon
Duralumin: 93% Alluminum, 7% Copper
Electrum: 55% Silver, 45% Gold
I might be wrong on some of these since I'm just going by memory.
To find out the percentages for brass, bronze and the others you could buy a sample from Brandon's site and send them off to a lab for testing.