First let's learn what keto really is because it's a terrible diet for maintaining or building muscle. A real keto diet is low protein, very low to no carbs and very high fat.
A true keto diet macro split is 5% carbs, 20-25%protein and 70-75% fat.
Don't do it.
Moderate to high protein, moderate fat and very low carbs is going to do you a lot more justice for maintaining muscle and your health.
There's no way In hell getting 75% of your calories from fat is going to be healthy.
Anyway, here's a list of cheap foods that will be good for a low carb diet.
Whole eggs, chicken leg quarters, ground beef, cheese, protein powder, Greek yogurt, heavy cream, nut butters.
Quality Fish and beef are expensive, but canned fish and pork chops on the bone are fairly cheap replacement foods.
Then of course you have vegetables, there's plenty of low carb veggies.
Keto breads are pretty gross in my opinion but there are some ok keto wraps that you can use as a pizza crust to make some fire flatbread pizzas.
Also, go to YouTube. There are plenty of ketotards who have gone to the extrema to find keto meal ideas.
Remington James is about as ketarded as it gets. This guy spent like 2 years on a keto kick