I am on a tight budget.
I’d recommend keeping it simple. When I go out for fast food, I usually go to McDonald’s. Here in Minnesota their McDouble is only $1.49, and it has two patties with a slice of cheese IN THE MIDDLE. This is KEY as then it is easy to take off the bun. I normally order 4. Yes, FOUR. Four McDoubles are approximately 68 grams of protein, 67 grams of fat, and the cheese is 6 grams carbohydrate. (1.5 grams per slice cheese). It’ll fill you up, trust me.
If you get braver, see if your McDonald’s will sell meat patties separately. Mine does. I buy quarter pounder meat patties for $1.29 each!! But I never order patties separate at the drive thru. I go inside when they are not busy because they find the order challenging. Be ready to explain.
The other cheap “fast food” is Chipotle. Get the burrito bowl (no tortilla), skip the rice, beans and corn salsa, and load up on everything else.
Perkins has real butter if you ask, and at least 5 low carb sides to choose from with their dinners such as green beans with bacon, grilled asparagus, a side salad, broccoli, or sautéed spinach.
(I get the pork chop, salmon, or tilapia dinner).
Any restaurant that serves breakfast will be cheap and low carb. See if you can negotiate an extra egg or bacon for leaving off the potato or bread. Most will. A loaded omelette always works too.
I have many more suggestions. I’ve been doing this keto thing for over three solid years now