i got mine at a junk yard for $25. the ac vents pull out (stick your fingers in there grab on and pull) and the trim ring pops off so its very easy to change. a knife, flathead screwdriver, or maybe just a fingernail is the only tool needed.
if you go to the dealership you can only buy the vent assembly, not just the trim ring. fx4 is a silver vent with chrome trim, xlt and harley are the black vent with chrome trim.
the vents are #8 in the pic and im guessing the price is per vent.
http://www.partswebsite.com/fordpart...imagecallout=8