Like anything I guess, you *can* spend a lot on gardening if you want, but it is by no means necessary. There isn't really any building required in most cases, and if there is (like you're on bedrock and NEED raised beds) you can perfectly well use scrounged stuff. If you save and trade seeds, and grow perennial things from seed or cuttings or tiny divisions, it needn't cost much for the plants themselves (unless you are planting an acre market garden but that's a whole different affair).
And as far as fertilizers, soil amendments, mulches, etc, the vast majority can be taken care of just fine by good compost, made from scrounged materials to whatever extent necessary.
The main thing that money substitutes for is TIME. As long as you are willing to supply the raw material and let things develop gradually, nature takes care of an awful lot of things on its own
I am certainly not criticizing anyone who spends whatever amount of money on their garden -- and I will admit I spend $300-400 per year, almost entirely on shrubs, trees and perennials, because it bugs me to live amongst 'nothing' and we are probably only going to be in this house for another 15 years -- but, it is not like spending money is some inherent requirement of gardening
Indeed, given that things both rot down and grow ON THEIR OWN, gardening is in principle one of the
less expensive things you can do
Pat