Lentil soup is great and inexpensive. I am making Icelandic soup/stew today. Had it when I was in Iceland last year and loved it. Also love tortilla soup. Here's a recipe for you. It's really easy and really good. I only use 1/4 teaspoon cayenne and usually use 2 quarts of chicken stock. My college age daughter always asks for it when she comes home.
http://www.grouprecipes.com/19742/the-best-tortilla-soup-houston-junior-league.html