Also it depends on your circle and negotiations skills.
Local economic issues aside...
Your paying people for the risk they take, when they grow or sell it.
Drugs being an underground economy, you pay less for a product, and more for a service. The service of reliability, quality control and availability are what people "sell".
Why would MM growers divert their product to the illegal market? That increases risk, and likely is less profitable.
Growers do not want to have to grow more to sell for less to make the same amount of money.
Well put keta-friend.
Also, if you buy from a grower, and he doesn't think your a tool, its likely going to be cheap as hell. I buy hash oil 1 gram at a time from my grower and I can turn around and sell that same single gram for twice the price, and people STILL think I'm giving them a good deal. (Disclaimer: I do not deal drugs)
So like kaywholed said, your not necessarily paying for the weed as much as you are for the distance it travels to get to you. The more people in between you and the grower, the more you will pay.