There are many articles about the 2011 UW-158 asteroid which will pass earth tonight. The interesting thing about this asteroid is that it is said to contain $5.4 trillion worth of platinum.
Many articles state that this asteroid is an X-type asteroid and is estimated to have 90-100 million tons of platinum in its core. The asteroid is said to be 300$\times$600m in dimensions, so this amount of platinum would constitute a large fraction of the asteroid mass.
There is also something worth noting. The current platinum price is about 32 dollars/gram or 32 million per ton. So the estimated 100 million ton would be worth $3.2\times10^{15}$ dollars which is almost 1000 times more than the estimated $5.4 trillion. So there must be something wrong, either the price or the mass, and that's why I ask this question.
Aside from that asteroid, there are obviously some types of asteroids with different metal content as percentage of the total mass. So what is the mass fraction of platinum/rare metals that we should expect in different types of asteroids ?