Find the lowest price for the Xbox One by using PriceGrabber and following seasonal deals from Microsoft. There is no gaming console by the name of Xbox 720; the Xbox One is the successor of the Xbox 360.
To make sure you only see the console and not any third-party products, do not use the search function on PriceGrabber. Click Video Games on the home page, and then choose Consoles & Accessories under Xbox One. Click Console on the left. In addition to finding the most affordable options, you can see bundles that include various games at lower prices.
Microsoft often reduces the price of the console during seasonal events such as Black Friday. Check the Sales & Specials page at Xbox.com to look for active deals.