$20 for a gram of plant matter is expensive. 28 grams in an ounce would equal $560, although you get discounts buying in bulk so it'd be more realistically about $500. $500 for an ounce of a plant, most other plants in existance aren't $500 per ton. It's an artifically inflated price created by the black market, but that's not what this thread is about.
The point was it's as cheap if not cheaper than cigarettes so adding cigarette tobacco in wouldn't lower the cost, if anything they mix it up to make it more powerful.