Costco. It isn't the absolute cheapest option all the time, but it is consistent quality and convenient.

You can spend considerably less than $600 a month on a keto diet from Costco (especially if you keep your meals simple.)

Chicken, ground turkey, hamburgers, ground beef, pork, etc are all very reasonably priced. Freeze anything you won't use in a few days to avoid waste.

They have a lot of keto friendly side dish and vegetable options. If you're cooking for 1, picking 1 type of lettuce/salad mix and 1-2 other fresh vegetable types per week should meet your needs.

Their heavy cream is $10 for a horizon organic at my costco. Very cheap compared to buying smaller bottles if you use it regularly. Eggs are also reasonable.

You can stock up on staples like coconut oil, nuts and seeds, mayo, etc there as well.

The kirkland protein bars are similar to Quest bars, but $18 for 20 versus the $24 for 12 you might pay for Quest bars from amazon.

If you're trying to eat a lot of steak for dinner, or wasting a lot of food, $600 will happen. But if you are shopping at costco (or a similarly priced store) and make good decisions you can enjoy good, satisfying food with variety for way less.