Formal education or not, I've explained everything as simply as I can. The price is fixed by the consumer, not the dispensary owners. It's perfect competition, if a dispensary decided to charge more than the standard, consumers would go to another dispensary, since marijuana at one dispensary is a perfect substitute for marijuana at another dispensary. If a dispensary decided to sell for less, they would gain a slightly larger market share, but may not even be profitable to stay in business.
If the consumer ultimately refused to pay $20 a gram, dispensaries would have to reduce the number of employees and reduce what they paid the grower for their product in order to get the product to the consumer at a price they were comfortable paying. It just so happens that $20 a gram is that price.