Human nature is such that we are willing to believe anything can resolve our problems and the more expensive, the more likely we are to believe it. After all if these supplements were cheap, they couldn't be any good, could they? Marketing history is littered with examples of products that were sold too cheap and failed. Burberry have just destroyed £28 million of stock in order to prevent it from being discounted and ruining the image they are trying to rebuild after it became a chav brand.
I work in a company that supplies raw materials to cosmetics makers and we sell large amounts of extracts, which are a 50/50 blend of water and proylene glycol with the plant material, e.g. dried plant petals or leaves, macerated in the mix for a few days, filtered and then preserved with a bactericide. This is added to a batch of product at 0.1% or less, which means 1 kg or less per ton of finished product. That's just over one drink bidon in an IBC of finished product. This enables the manufacturer to claim that their product contains extract of plant X and charge huge amounts of money for what is basically thickened water with some surfactant, sold as shampoo or foam bath or an emulsion of water and oil, sold as cream. If the extracts aren't mentioned on the label the consumer will move on until they see a product that does claim extracts. You could never prove any benefit to the skin or hair; the benefit is entirely in consumers' minds.