If you can grow outside with natural light, that is the cheapest. But you have to plant in May. Way too late this year, unless you are in the Southern hemisphere, or in the tropics. In the tropics, just get some autoflower seeds and you can grow anytime. A bag of compost, a small bag of bloom fertiizer, and a pH meter (about $10 on Amazon.) Adjust your pH to 6.5 with vinegar, if you need to. A bottle of spinosad keeps the bugs away. All from home improvement stores.
As soon as you go inside, you need to buy or build lights and then pay for electricity to run them.