My parents started out very poor and climbed into the middle class by the time I got to high school. The extended family is still poor, so I'm still exposed to how they think about money. I think the mindsets can break down into three levels, but not cleanly.
Poor: You may have to make decisions about which bills to pay or which needs to meet, and being able to pay all your bills and eat is considered a moral virtue. Money is naturally transient, because your obligations have similar weight to your income, and having excess money in your bank account is weird, unnatural to the point that your instinct is to spend it before it goes away. If you use a budget, line-items are on the size of a grocery trip. You're more likely to budget small purchases than large purchases. If you are trying to build savings, putting money into a savings account is another bill.
Middle class: Your bank account is the intermediary between your bills and income, with a mean and a natural trajectory. If your account's trajectory is positive, your natural instinct is to spend it, either on better lifestyle or a large purchase, very occasionally savings. If you have a budget, components are like "monthly food" and you deal with going over-budget by cutting a few more corners next month. Savings are still another bill. Putting the right amount into your savings account to retire in 40 years is a moral obligation.
Capital class: Interest on your earnings is your primary source of income. Your rate of spending must be lower than the rate you earn interest; to do otherwise is dipping into capital, which is morally wrong. Any interest you don't plan to spend is folded back into your capital, giving you a small, permanent raise. I don't understand this class too well because I'm still working on entering it.