As Faustus does the whole should-he-or-shouldn't-he-deal-with-the-devil calculation (here's an idea, Faustus: you shouldn't), the wealth that such a deal can bring him factors considerably into his fuzzy math. He knows that, with Mephistopheles's help, he can get his grubby hands on the treasures of exotic places like India, Asia, and the Americas. Plus, he could use his considerable power to cheat peasants out of their money and possessions. Beyond demonstrating the cravenness of his character, Faustus's desire for wealth and his willingness to sell his soul to the devil shows us that in Doctor Faustus wealth and salvation don't exactly go hand in hand.
Faustus, Valdes, and Cornelius are all about exploitation and destruction. When they imagine the wealth they'll get with their magic, they don't do anything nice with it, which shows that ill-gotten wealth is bad news.
Faustus's use of magic to deprive peasants of their money emphasizes the importance of honesty and transparency to functioning capitalist economic exchange.