I don't know about the costs, but here in Brazil the turnout for the second round isn't much lower: for instance, for the 2018 federal elections, the turnout wa

I don't know about the costs, but here in Brazil the turnout for the second round isn't much lower: for instance, for the 2018 federal elections, the turnout was 79.67% in the first round, and 78.70% in the second round (source: official results website). But that's probably because voting is mandatory (except for voters younger than 18 years or older than 70 years).