Tablet PCs, iPads and mobile phones all include facilities to browse the mobile Internet. The costs of mobile Internet access may become extraordinary, however, when the data limit is exceeded or when the user is roaming abroad without a roaming data plan. Since users may see advertisements as unwanted traffic, they can be confronted with a bill of which a significant part is caused by data they did not ask for. The goal of this paper is to investigate the amount of data generated by advertisements when browsing, and the potential costs of such advertisements for average users. Our analysis shows that web advertisements take up 7-9% of the total data traffic, depending on Flash usage.