Ford Discounters has them for $65.00 for the OEM replacement lamp/housing
Check out Tousley Ford, Steve might be able to get you an awesome deal!
I would also check out RockAuto, sadly I didn't find a high mount brake light on their site(they are usually very cheap)
Check out Speedway recyclers as well, they are the largest site for junked Foci:
Also, go to your local junkyard, you should be able to get one there for very reasonable too!