Does Maya have any method for working out area or volume? If it does, area in metres^2 x thickness (0.005m) will give you your volume in m^3, then multiply volume by the density of steel (7850 kg/m^3) to get an fairly accurate estimate of mass.

If you don't have a tool for calculating area or volume in Maya, you could either import it into a CAD program that can, or break the armour down into squares and add the areas up, then proceed as before. Otherwise I can't really give an estimate without measurements on those panels.