There are VERY specific rules (certainly in AUstralia, but im pretty sure internationally) in regard to advertising space and political advertising during an election.
Rule one - (so as to be un biased ) All advertising costs are at "rate card", so full price... like your hotel room that no one EVER pays the full rate... except for politicians. In the advertising world No-one ever pays full rate card rate in newspapers.
Rule two - there is (supposed to be) a very clear deliniation between editorial and advertising, with "Advertorials" banned during election campaigns.