“Extremely” is a matter of perspective. A handful of random undifferentiated asteroid regolith is going to have a higher ppm of, for example, gold than a handful of random Earth gravel, but it is still going to gave a much, much lower ppm of gold than a mined gold ore on Earth. You could mine gold on Earth by taking billions of tons of random rock and sorting through every atom for the gold, but it would be hugely expensive. That is exactly what you would have to do to mine it from an asteroid. There might be a higher ppm of gold, but you have the extra complications of trying to do it in space.
Citing a post by myself:
For many of the elements that are rare on Earth’s surface, those geological processes that concentrated them concentrated them down to the Earth’s core. In a differentiated asteroid, those same elements would be concentrated into the asteroid’s core. Luckily, most of the asteroids large enough to have differentiated into a core and mantle/crust have been blasted to bits and those pieces of core are floating around naked.
According to this chart the average concentration of gold in the Earth’s crust is 0.0031ppm. According to this PDF, the concentration of gold in meteorites varies, but certain iron meteorites (called “siderites” in the old study) contain up to 10ppm gold. In undifferentiated meteorites, the gold percentage is still much higher than the average for Earth’s depleted crust, with chondrites having from .1 to .37ppm. According to this page, the highest grade gold mines are at around 1 to 3ppm gold.
So asteroids range from “lower concentration than a rich mine but higher than a random handful of Earth’s crust” for stony meteorites to “similar to or greater than the gold concentration for the highest-yield mines” for iron asteroids. Plus other precious metals. Whether or not it would be possible in the short or long term to mine asteroids is questionable, but there is gold in them there floating hills.
At the rate of 1 ppm, you would need to sift through every atom of more than 30 tons of asteroid material to collect 1 ounce of gold.