Depends on the carrier and the plan. I gave my US mobile to my mom as I travel frequently out of the country. I have a plan with a carrier in Singapore where I've been working and the rates are more than affordable and well worth it considering the lack of hassle in pre-paids.
As I originally said, check with your carrier and find out the rates BEFORE doing anything. Paying a sum like that would be stupid. But not all carriers have the same rates.