Dried legumes and canned fish are your best friend right now. Beans, lentils, chickpeas - they cost basically nothing and have serious protein. Canned tuna or sardines are also cheap and come ready to use. Nobody talks about this but they really fill you up.
As for eggs and Greek yogurt, I agree they work well, but combine all of it. A boiled egg in the morning, some yogurt in the afternoon, some lentils in your main meal - you'll have protein all day without spending anything. Supplements? Only if you want them and have money to spare. Real food stays cheaper.