Are you interested to know that how much data FaceTime over 3G actually uses? To answer this question 9to 5 mac has conducted a 5 minute FaceTime call with lots of movement on a jailbroken iPhone 4 and in total it used 14.7 MB of data. It uses 3MB data per minute.
Before 3G Face Time Call
After 3G Face Time Call
The data usage increases after implementing face time video call over 3g.