There are two primary reasons why top-grade medicinal pills command such an exorbitant price.
The first reason is that their efficacy is several times greater than that of low-grade pills; taking a single one yields the effect of taking multiple pills.
The second reason is that the higher the grade of the pill, the less medicinal toxin it contains. As the saying goes, every medicine is three parts poison, and this rule holds true even for pills in the immortal cultivation world.
Chapter 32