Not quite $30/month, but someone decided to limit himself to $2.5 per day for 46 days and the results look delicious https://futureboy.us/blog/twofifty.html

I skimmed that post and it uses the classic technique of "if I have to buy something for 10$ but only eat part of it I count the amount it costs me for that day" - that only works if you have the money to buy in bulk, which is not the case for the majority of people who get dished out the "you can eat for almost nothing!" advice.

If you limited your diet to rice, beans, oats, and lentils bought in bulk you can probably get close.

And have a stove, pots, and a refrigerator to store what you cook.

And don't mind missing a ton of vital nutrients (Vitamin A, D, Calcium...)

search google, this experiment has been done a few times. You can get the required caloric intake for an adult for under $30 per month.

it's not the tastiest of food and it'll be mostly the same, but it can be done.