Business Week has an in depth study and article on The Poverty Business.

Federal Reserve data show that in relative terms, that debt is getting more expensive. In 1989 households earning $30,000 or less a year paid an average annual interest rate on auto loans that was 16.8% higher than what households earning more than $90,000 a year paid. By 2004 the discrepancy had soared to 56.1%. Roughly the same thing happened with mortgage loans: a leap from a 6.4% gap to one of 25.5%. "It's not only that the poor are paying more; the poor are paying a lot more," says Sheila C. Bair, chairman of the Federal Deposit Insurance Corp

It's a very good article, going into depth on just how badly those who are scraping buy are even further victimized by these various credit agencies.