3G or 4G use overseas is an expensive luxury, but there are a few ways to reduce the costs.
Data roaming can run up huge bills, but you can turn it off. Open Settings then select Cellular (or Mobile Data in some regions) followed by Cellular Data Options. From here, toggle Data Roaming to the off position.
You’ll still be able to access the Internet from any Wi-Fi connection.
If you do need to use the data connection, leave Data Roaming turned on but avoid using apps such as Facebook, Mail, and Maps for long periods of time, and avoid streaming videos or music, as these can use up large amounts of data.