Protein sources which are not too expensive

If you’re looking for price friendly protein you might consider beans and lentils.

I would imagine those are readily available to you in India? I know I cook a lot of Indian dishes as a Vegetarian and legumes and pulses are a huge part of those recipes.

Hi avikalshukla935
Fairly affordable sources of protein are pulses (beans, lentils, chickpea etc). You also have grains such as buckwheat, quinoa.
Eggs and fish are very good sources of protein and vitamins, not to speak of healthy fats which your body needs if you intend on increasing physical exercise in order to gain weight (muscle). Essential fatty acids help your body reduce inflammation after workouts and assist in the healing process of broken muscle.
Protein yoghurt, cottage cheese and quark are also excellent sources of protein, have live bacteria cultures which contribute to gut health and help both with weight gain and muscle building.
A glass of milk with a banana and a spoon of peanut butter (make them into a smoothie) before bedtime will help you gain weight, keep your blood sugar levels under control overnight and help you sleep better. Add some protein powder in there if it is affordable for you.
Remember to allow your body at least 2 days of rest between training days when weight training for the muscles to repair in order to retain weight. Also make sure you meet your daily calorie intake.