The density of FR4 is 1,850 kg/m^3. If we assume a PCB area of 12*50 = 600 mm^2...
For a 1.0mm PCB: 600 * 1.0 * 1.85 = 1.110g For a 1.6mm PCB: 600 * 1.6 * 1.85 = 1.728g