There are two basic ways to buy a used car cheap, and one good reason: To save money. It isn't just that they cost less to buy initially. The research shows a four-year-old car can cost 40% less over its lifetime. That's including the costs of gas, oil, repairs, insurance - everything. This is why almost half of the millionaires in the United States buy used cars, according to Thomas Stanley and William Danko, authors of "The Millionaire Next Door."