Sam Smith, Does it have to be only Delta flights? I just Googled 'msy to dfw flights' & found other airlines offering flights at least 40% less than Delta! But, just like Chinemelu Elonu said as well as depending on the day of the week or year since some events are busier than others (Thanksgiving, Christmas, etc.), your best bet may be searching Expedia, Priceline, etc. Like anything else you'd buy, you have to shop around to get the best deal!
That is something to all Priceline, Google Flights, Expedia and many others
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