Cuttings from stuff you can find growing around is free, for pots/soil you can just dig it up somewhere if you want, use an empty bottle as a pot if necessary. Pretty much everything I am growing currently is perennial too.

Got an allotment now which is around 125m³ and costs like £40/year. Spent a bit on getting a bunch of different things to try to see what goes well and so far the things that are growing well are kale, walking onions and rhubarb. It would cost well over £100 to buy enough of those to cover the plot I have, or I can propagate them to get more over time for the much lower initial cost of buying just a few of each plant. Of course it will take a bit longer to get a high amount of productivity from it, that said the kale seems to be growing pretty fast now and I could probably multiply it by 4 in just a few months. Rhubarb and onions probably going to take a bit longer, rhubarb is a popular plant so I might be able to get some from other people around me for cheap/free. Could offer some walking onions, don't see many people growing those so its something more unique.