Re: how much gold, platinum ... present in asteroids is still completely unknown
Scroticus Canis, precisely the reason Mercury might be a good place to prospect if energy to change orbits was cheap enough. It seems to have lost its crust and maybe mantle due to the energy of impacts. NASA did well with Messenger.
On another suggestion, NASA have chosen an apparently metallic asteroid which may be a core remnant for a mission. Current energy technology makes space mining hopelessly uneconomic for the foreseeable future unless fusion reactors can use He3. Then it might be worth mining the Moon for helium, not metals.