Foraging for wild food is a smart thing to do, though, not only for your budget, but for your health. As a rule, wild food is healthier than domesticated crops, delivering up to 10 times as much nutrition in the same amount of food.
There are two things you need to be careful of. One, make sure you know what you're gathering. There are not many plants that are deadly poison, but be sure you don't gather the few that are.
The second thing is to never forage in areas that have been sprayed with any kind of chemical. Many city lawns have been treated with herbicides to keep down weeds and pesticides to kill insects. Don't pick anything from them.
Where to find free fruit
Not only can you forage for vegetable plants, there are plenty of fruits that can be foraged, in the wild or not. How many apples does a mature apple tree have? More than an average family can use! It doesn't hurt to ask when you see a tree loaded with fruit.
There are fruit trees, brambles and bushes planted as ornamentals on college campuses, business lots and city greens. Check into them; why not? The birds won't eat them all. Some of them may have been treated, but they are not always.
Foraging for vegetable plants is even easier
If you have a back yard, don't put anything dangerous on it. Leave a small part to grow "weeds" or deliberately plant them in a contained area. If you're in a hurry, you can buy seeds from many plants, including dandelion, purslane and lambsquarter.
Don't gather anything from a public park, as it is nearly always treated for weeds as well as insects. You can, though, gather seeds from there if you're careful to be there at the right time and plant them the next year.
You probably shouldn't gather plants from roadsides, either, unless they're dirt country roads that see little traffic.
Get a good book for your area, since different plants grow in different areas. Here, I harvest lambsquarter from mid spring to fall, dandelions in the spring and again in the fall, purslane from late spring through at least the first frost, but your area may be different. Find out and go gather some free food!