There are a few jailbreak apps on Cydia that let you use FaceTime on 3G: 3G Unrestrictor, My3G, and FaceBreak. If like me you’re on an unlimited data plan, data usage is not a problem. If however you are on a capped data plan, using FaceTime over 3G could take you overboard.
A few days ago, I made a FaceTime call over 3G for 10 minutes to figure out how much data FaceTime uses on 3G. As you can see on the image above, I sent about 15 MB of data and I received about 12 MB. I think I sent more than I received because I was moving around a lot.
Total data usage for a 10-minute FaceTime call on 3G: 26.5 MB. That’s about 2.65 MB per minute.
Obviously your mileage may vary but these numbers shouldn’t be too far from what a FaceTime call over 3G will cost you.