well, unless you can emulate the climate and growing conditions, its useless to grow them. if they dont get proper care from day one, the potency of the leaves are decreased a LOT. About 297 grams of dried leaves yields about 1 gram of cocaine. in the wrong climate (if you cant grow coffee, you cant grow coca, plain and simple) you will be lucky to get a gram of cocaine with somewhere near 500 grams of leaves. I use to grow them for such reasons and i was taking up to 480-500 grams of leaves to produce a gram of cocaine.
doing a garden is useless as the plants take a while to grow and your looking at 20+ plants to get a some-what decent yield of cocaine.
at the cheapest, your looking at $50 for 10 seeds, about $200 for special soil, lights, etc, another $100 for all the supplies to extract the cocaine, and a shit load of wait time as coca tends to take a while to grow. not worth it at all. your looking at years of growing to produce a couple grams of cocaine.