Just an FYI- I just hit an OEM Ford parts website and priced factory exhaust manifolds and gaskets at about $500. If you want headers for performance purposes, fine. But if you are looking just to replace defective parts with the most economical alternative...

I'll bet that a fair chunk of that $1,700 estimate is for labor. I had a 4.6 triton in my 1999 Expedition, and component accessibility wasn't its strong suit.