>Is it expensive?

$125US per exam. Can vary by country.
Prep materials: $0 for using only MSDN or Wiki or using the book $39US-$49US

>Difficult?

I found the VFP exams more difficult that the 70-100, the VB Desktop exam and the NT workstation exam that I took. This is a good thing.

>How does one get prepared?

It is possible to prepare for the exam just using the VFP help, the MSDN and the wiki but it will save you tons of time getting the book (okay okay I'm biased). As well, I found that studying with a group kept me focused since I didn't want to let down the group.