On all of Delta's domestic flights (specifically, anywhere within the 50 states plus Puerto Rico and the U.S. Virgin Islands), baggage fees start at $30 for a single checked thing in the economy; a second checked bag costs $40.
That changes assuming you're flying internationally. Out traveling to Rome, for example, the first bag is complimentary, and you'll pay $100 for the second bag. Ditto for flights to Beijing and Tokyo, however for other international destinations — like Delhi, Cairo, and Muscat — you're allowed two free checked bags. The exact charge depends on where you're flying and can be found by using Delta's online baggage expense calculator.