Does FaceTime cost If the person I'm FaceTiming is abroad?

I live in England and I usually FaceTime a girl that lives in New York on my iPad, because my charger is being a dick I wanted to know I FaceTiming from my iPhone 5 would cost? I've heard it's free over Wi-Fi but what would happen if I lost the Wi-Fi and was on 3G? Would I charge me because of that. Also I'm on t mobile!

Yes it will

Imessage and facetime can both be used free over Wi-Fi or even internationally, you do not need a monthly contract to use these. Many are not only doing this with phones but many do this on ipad and ipod touch also.