Due to low seating capacity in both ballparks, Cleveland Indians v. Chicago Cubs in the 2016 World Series may be averaging fewer than 40,000 fans per game.
Wrigley Field, the Cubs park, is old but the Indians park is pretty new but it's one of those clever modern parks built to have the features of parks from 100 years ago, including exposure to the elements, like rain, wind and cold. It's also small to keep tickets scarce and expensive.
After some common sense checking it appears that the most recent World Series with a lower average attendance is probably 1946: Cardinals v. Red Sox. The Cardinals were still playing in their old small park and Fenway Park in Boston had yet to add those monstrous seats above the 37 foot high Green Monster wall in left.
Of course, Wrigley Field, where the Cubs play, was the last park to not have lights, which were added in the 1980s to accommodate night playoff games on television.