A Beginners Guide To Growing Marijuana Indoors. Educational purposes only - For many cannabis users, the cost of the plant becomes either an unwanted nuisance or at worst, an impenetrable barrier between them and their medication. The cost of growing a cannabis plant is very low indeed. Rather than the inflated £10/$15 per gram prices on the black market, in coffee shops, and sadly even many dispensaries, the actual cost of growing a cannabis plant is closer to just a couple of pounds, even with several plants on a really small scale.