You may have run into this problem, one headlight dies on low beam, but the headlight checks out good when tested.
VERY simple repair:
Remove the diagnostic display on the console by simply pulling it to you, underneath it on right side is a small black module held in by 2 screws, remove and disconnect from harness. Open the case by pulling on tabs, remove the circuit board, turn over, look for bad solder joints (usually on one corner of the board near the connector) a tiny bit of solder on the bad joint, re-assemble, you now have headlights again.
Cost: 0 dollars

It's that easy.

 

The "hardest" part of the repair is re-installing the 2 mount screws that hold the module.
The diagnostic center will now quit beeping at you with the "headlamp out" message, and no wires splices from the operating headlight to the non-op one (which will still give you the warning message)...............AND if you can find a OEM module from Ford, $78 is pretty steep, J-yard specimens will likely be bad too.