Organics are by far the cheapest way to grow weed but you will need some raw materials to work with. Thankfully you don't have to get everything all at once to start off....Aquire what you need as you go.
Start by getting a bag of good organic garden soil. Cannot go wrong with something like ocean forest or happy frog or both but any decent soil with good drainage will work. You will also need a bag of earthworm castings. That's all you really need to start off your plants.
Ffof is like $35 for a 1.5 cu ft bag which will get 2-3 plants through seed to harvest. Unco ewc is under $20 a bag and is enough for probably 2 grows. Get some Jobes organic spikes for bloom phase; $8 for 50pk will last a year or more. Charlie's compost chicken manure is very good fertilizer at $18 for 10 lbs will last a real long time. Great for mid veg through flowering.
Finally, get some D-lime for recycling the soil after your first harvest. You can aquire more amendments to recharge the soil like crushed oyster shell, fish bone, kelp, and neem meal as you go.
Having all this stuff on hand allows you to also make aact teas, diy spikes, and other ways to keep the soil active and the plants happy. A small investment up front makes long term soil growing very cost effective. Plus organic cannabis tastes the best. You'll get a more exotic end product for very little effort; all you do is just water your plants.