3G or 4G use overseas is an expensive luxury, but there are a few ways to reduce the costs if you have a cellular iPad and plan to use it in another country.
Data roaming can run up huge bills, so turn it off from Settings then Cellular Data (Mobile Data in some regions.)
If you do need to use the data connection, leave Data Roaming turned on but avoid using apps such as Facebook, Mail, and especially Maps for long periods of time, as these can download huge amounts of data.