It is difficult to say, as every network has different plans, bundles and add-ons, but as an example, O2 charges ?3.07 per MB when roaming in Europe and ?6 per MB for the rest of the world, while Vodafone charges ?1 per MB up to 5MB, then ?5 for every additional MB after that in Europe. For the rest of the world they charge ?3 per MB up to 5MB, then ?15 for every MB after that.