Ganja is another name for cannabis. Cannabis is easier and cheaper to grow and prepare for use than tobacco so if the tax rate was the same, you'd expect it to be about the same price. And the equivalent weight of cannabis would last even a very heavy user longer than a tobacco user so it would cost much less than smoking tobacco. Of course the tax rate could be higher or the producers could profiteer so it could easily be more expensive by weight. Easy enough for people to grow themselves though. Pretty much.