In addition to what others have said, I'd also like to point out that there is a difference between personal wealth and business capital. While Uncle Owen may not make enough on his farm to have frivolous disposable income, that doesn't mean the farm as a business is bankrupt.
Take a look at these tractor prices. They range from $300 to "500, which makes them comparable to luxury and sports cars in price tag, yet farmers who own them can't afford a car like that. The business has capital and it buys things because they are useful. But a sports car isn't useful for farming. It may be that the droids are quite valuable.
