As others pointed out herbs might be the only thing that come close to being cheaper.
Speaking oft herbs - Weed might be the only thing that fits your requirements thats actually cheaper this way.
I grow several herbs and it costs nothing after the initial cost to buy some plants - and that can be avoided if you know someone else with herbs if they are willing to share some cuttings or seeds.
As others pointed out herbs might be the only thing that come close to being cheaper. Speaking oft herbs - Weed might be the only thing that fits your requirements thats actually cheaper this way.
I grow several herbs and it costs nothing after the initial cost to buy some plants - and that can be avoided if you know someone else with herbs if they are willing to share some cuttings or seeds.