I think this article "How to Get Enough Protein on Tight Budget" gives some good suggestions on how to supplement protein intake with foods other than legumes such as long beans. I agree eating meat (such as fish and skinless white poultry lean meat) occasionally is recommended to get some animal protein with amino acids. I learnt that canned sardines are healthy and inexpensive. I sometimes eat canned sardines, and it's one of my favourite dishes. It is also high in calcium and good for the bones, and contains iron and omega 3 fatty acids, as explained in the website below.
"Canned sardines are packed with many nutrients. Just one serving -- a 3.75 oz.
water-packed can -- contains 19 g of protein, helping to build and maintain lean
body mass. It also contains important minerals, providing 20 percent of the daily
value for calcium and 10 percent of the daily value for iron of a typical 2000-calorie
diet. Sardines are an excellent source of omega-3 fatty acids, the type of fat that is
protective against heart disease, stroke and other medical conditions."
I also learnt that yoghurts are good source of proteins and they have the added benefit of probiotics
which is great for digestive health.
"Real yogurt is made from RAW dairy products and contains lots of good protein and comes from the whey in milk."
"8. Yogurt is an excellent source of protein. Plain yogurt contains around ten to fourteen grams of protein per eight ounces, which amounts to twenty percent of the daily protein requirement for most persons."
So, I think these are some healthy and inexpensive sources of proteins to consider.