I think txt messages are different than voice calls in that they are not subject to international rates. Txt is data traffic carried that doesnt' get 'terminated' by local telephone companies like voice calls.
I'd be very shocked if they had a different rate for US txt messags vs international txt messages.
they are different domestic rates are $.20 send /recieved and international are $.25 set /$.20 recieved
http://b2b.vzw.com/international/Text_Messaging/index.html
