This info is not info that companies would spend time and money to list details of. If you wanted to though, you could always dismantle a computer weigh the like plastic into categories, weigh like metals, and then send samples if the plastics out as well as samples of the circuit boards out to have them use a spectrometer to give you a graph that shows every detectable element.
Plan on having about a million dollar budget to have the numbers as precise as possible.
http://en.wikipedia.org/wiki/SpectrometerMost people for say a school project would get say 10 dead computers from what may be going to landfill or recycle, and then document the strip down and weights of the proportion of plastic to metal in the computer. If they wanted to take it 1 step further and had proper PPE they could dip the circuit boards into an acid solution that would eat the boards and metal, and then isolate the metal from the fiberglass and other non metal impurities and be able to state that there was 3.8325 ounces of metal copper/gold/lead mixture in the boards and from top side components and 15.0932 ounces of fiberglass and random partially eaten components.
Based on older and newer computers one could even show how computers over the last say 20 years have declined in weight and metal usage and increased in plastic usage if they have a sample of 10 systems going back to a Pentium 60Mhz bought in 1994 to date etc.
Most may not take your post seriously as for it is far from the norm that we get here, however, I wanted to help if this is a real question and suggestions on figuring out weights and measurements of materials and costs associated with being very precise, vs for less than $100 you can get a rough assessment without having to know that there is 1.2 grams of zinc in the Galvanized Steel case.
If only calculating for zinc. You could always calculate the area of the metal of every bend and radius or hammer it flat to be dealing with 2D measurements of area, and then find out the average amount of zinc used to galvanize a 1 x 1 inch square area and then with that number get a pretty good estimate, although not actual, measurement of actual zinc in the metal of the case. Contacting a specific manufacturer you may be able to have them share how many microns the thickness of the zinc plating is etc.
