FWIW, HD tow is included in various Ford packages that one might not expect to find it, including the Convenience Package, Special Edition Package, Texas Edition Package, Stealth Edition Package, and possibly others. So if shopping by window sticker alone just be aware the full content of these packages.
Ford periodically does package discount credits. If you are on the fence about the $3995 Special Edition package, know that it includes the $1570 HD tow and currently Special Edition has a $1060 credit, meaning you effectively can get some nice bonus options in addition to your HD tow for a meager $1365.
As a stand-alone option HD Tow is $1570, roughly 2.5% of a typical Expedition MSRP.