I believe only 100F is available in America, while 100 (no F) is only available in Japan. The reason the 100 is so much more expensive is probably because they have to import it from Japan. However, I think I've heard that Fuji is discontinuing Velvia 50 and that they will soon be making 100 available in America.
From what I've heard, 100's colors are more like 50's, while 100F's are more subdued. I don't have any personal experience with any of these, though, so I can't say for sure.