Yes extra data on postpaid is $10 per GB and many people including myself call that disgraceful. Unfortunately the delay in the data usage meter means you are often playing Russian Roulette with your finances.
... and yes anything that streams video in either direction can use data at a rapid rate. Unless you have a generous data plan and you understand how the delayed data usage works I would be extremely cautious.
Edit: I should say that FaceTime itself costs you nothing as it simply uses the data that you already have. Its the speed at which it uses your data and the consequences of exceeding your data limit that is the problem.