they don't have to buy food, or water that much, heat either, Esme is an architect, carlisle is a doctor, and they save up money, plus alice can predict stock markets.
the cullens are soooo rich becuz dr.cullen has been studying medicine for over thousands of years and over the time period hes accumulated a lot of money thts wat i think
You see... Alice can predict the stocks (stock exchange - companies have what it's called actions, depending on the value of the company the actions will be worth more or less money!) and like that she can see which company will have higher or lower stocks! As such they can buy actions and then sell them, making money! Plus, Carlisle being a doctor brings a lot of money to the table:P
With Alice's visions and Carlisle's job, they are rich for centuries:)