Wealthy and poor individuals have limited budgets relative to their desires.
Every decision has an opportunity cost.
A decision to buy more French wine this month is also a decision to purchase fewer restaurant meals. Economists theorize that each consumer spends her money to maximize satisfaction or utility. We cannot measure utility directly, but we can find out how much you value a Starbucks mocha relative to a movie ticket. If you would give 3 mochas for 1 movie ticket, then the utility of a movie ticket to you is 3 mochas. This method is cumbersome so economists measure your utility of a movie ticket by looking at how much you are willing to pay for it.