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The worm that we mention in the pharmacy scene of the program (that sells for $8,000 for a pound) is actually the larva of a moth. It is what the dried larva contains that is interesting to Chinese medicine. Larva common to the Tibetan Plateau lives just under the surface of the ground. In a miniature enactment of Invasion of the Body Snatchers, the larva is often infected by Chinese caterpillar fungus (Cordyceps sinensis) 冬蟲夏草 (dōng chóng xià cǎo). The fungus kills the larva, takes over its body and continues to grow out of it until it is 3–10 cm long. see image

It has been used for centuries in Tibetan and Chinese medicine, but is more highly valued by the latter as an aphrodisiac, anti-ageing aid, and a tonic for the kidneys and the lungs. Its fame and consequent value has skyrocketed in the last decade.

More info:

Caterpillar fungus

In Tibet, a worm worth its weight in gold

Cordyceps sinensis Medicinal Fungus: Traditional Use among Tibetan People, Harvesting Techniques, and Modern Uses 

Drugs.com