Example one: Lettuce
If you buy a package of lettuce seed from the dollar store, a container from the same store (or recycle a container), and soil (purchased or from your yard). Find a window and plant. If you grow the leaf lettuce type you can keep cutting it. Your cost: $3 plus taxes for organic (you didn't add any pesticides). Cost to buy organic lettuce one cutting from a package is $5.99.
Example two: Peppers
If you purchase a package of seeds with say 25 seeds @ $2.00 or $.08cents a seed, make a seeding pot from newspaper see a previous blog on how to, and then some soil. Your cost is per plant is less than .25cents...to buy a pepper in a store can be easily $1.00/each.
If you start to share seeds or save your own seeds you can save even more. The added advantage to all this is that gardening is rewarding from both a physiological and psychological point of view. You don't need much space or money - just a willingness to give a little time. I promise it will be well worth the effort - especially if you can eat your own peppers and tomatoes in winter!