To answer your question, I'm not sure. My food expenditures per month vary widely, depending on whether I'm stocking up on things, whether I'm traveling for a week, etc. I probably spend around $300/month on average, but that's without doing a whole lot to try to save money (which is something I need to start working on.) Having said that, I'll throw in my two cents on the things I do to cut costs:
Great advice here already. Regarding Costco, I just made my first trip there a couple of weeks ago, and I'm still undecided as to whether it's worthwhile. I am also a plant-based eater, and I love to make 95% of my own food. My boyfriend has a free Costco membership through work, and I don't think I would have spent the money on a membership if I'd had to buy it myself. Also, it helps to have a really good handle on what things cost elsewhere before you go to Costco because you're right, there are some deals there, and then there are some things that are not deals at all. I did stock up on some basic staples that will keep for ages, like brown rice.
Also, my freezer is my best friend in this endeavor. Of course I freeze leftovers in individual portions, which not only minimizes waste but also makes life really easy. I also freeze just about anything I can in terms of raw materials. Even fresh spinach can be frozen; all of the spinach from one of those 5oz boxes can fit in a Ziplock bag, which I then just roll up from the bottom, compressing the spinach as I go, so I end up with a tube of spinach that fits easily into the freezer and can be used for smoothies (or maybe soup?). This allows me to stock up on spinach when it's on sale. Works for kale, too. I also store flour, flax seeds, chia seeds, etc. in the freezer because they last longer that way; again, that means I can buy them in huge quantities when they're on sale. (And in case you didn't already know, only grind flax seeds just before you use them, or grind just a small amount to keep on hand, because they go bad more quickly when ground.) Oh, and I buy lemons and limes when they're on sale, juice them, freeze the juice in ice cube trays (measure yours to be sure, but mine are exactly 2 tablespoons per cube), then pop the cubes out into a Ziplock bag. Easy cooking later on, and cheaper! (Also zest them and freeze that!)
And... dry beans! Never buy beans in cans! Dry ones are so much cheaper, and it's not a big deal to cook them as long as you plan a day or two in advance so there's time for soaking.
Finally, I've been getting into gardening. Nothing better than fresh, organic, almost-free food! My boyfriend started it, and now I'm hooked. He grows all sorts of things, but regardless of your situation in terms of growing space, even if you can just have a potted cherry tomato plant and a potted basil plant, that will make a world of difference. I made a ton of pizza/pasta sauce this summer for almost nothing and stashed it away in the freezer in Mason jars.
Oops, one more. If happen to find yourself in Arizona for another reason, this program is fantastic -- or maybe there's something similar closer you.
http://the3000club.org/wordpress/marketonthemove/