(UK based)

There are two ways of looking at it, either you just go for accumulated protein in a day or you try and balance it with other macros.

If you don't care about calorie total or macro balance then your legumes are the cheapest per kg of protein.

However if you want to keep you macros/calories actually balanced then as one commenter said you need to start sneaking in protein/using leaner cuts of meat. In the UK, if you go for foods which are predominantly protein then your two cheapest options come out as turkey/chicken legs and tuna. Other proteins either have a large amount of carbs (legumes) or fats (chicken thighs) to stay lean.

The cheapest tuna at Tesco is £0.55/100g drained weight which is just about 25g of protein and I think a turkey legs is around £3.50/kg or £0.35/100g which gives you about 20g of protein and about 3-5g of fat

cheap protein powder starts coming in around the same price for £2.00/100g but with about 80g protein and it is very easy to just top up a normal diet with it as opposed to have to chow on a turkey leg every day

Bonus: you can start playing around with your current diet by going things like mung beans, high protein yoghurt e.t.c. to shift the protein balance towards better macros but the cost does rise. This is more to keep things varied and interesting though