Megan McArdle recently posted about how poor people get forced into higher-priced alternatives. She credits that observation to the book, Nickel and Dimed: On (Not) Getting By in America
Basically, poor people do not have enough money saved up to make big purchases or investments. People from more affluent backgrounds will have money saved up or will have family from whom they can borrow the money. Without the extra money, poor people can not get the better deals that other people do.
For example, poor people cannot invest in an expensive and reliable car upfront, but will instead have to buy a cheap car and constantly pay for repairs. In another example, poor people cannot afford the deposits on apartments, and have to instead move into temporary residences with higher rent. Poor people often cannot buy products in bulk, which more affluent families do to get better value for their money.
Simply put, poor people can not make cost-saving investments because they do not have financial savings or rich family and friends.