Biggest costs of life are housing, food and transportation. With one person staying at home, both the cost of transpo and food go down (because: one less person commuting to job, everything can be home cooked and the person staying at home have time to shop smartly and save a lot thanks to it).
This lives housing costs. Here, it's all about location. In many areas (assuming renting), you can have a house or apartment suitable for a family of three for less than $1000 a month.