40 tons referrs to the mosaic, not the gold. 40 tons of gold in current value is 4 billion dollars.
The mosaic was created for over a year in Russia, during 2016 and 2017. It was then cut and transported by special trucks to Belgrade. The total weight of the mosaic is 40 tons and it was placed on the dome from May 2017 to February 2018.[76] The 3,289 m2 (35,400 sq ft) mosaic installed at the eastern wing section, the side of the altar covers 3,289 m2.
It is indeed an impressive and beautiful building.
