DDT does not have the lowest published rates.
Eagle Market Makers starts at $0.40 a side plus platform fees.
Advatage Futures starts at $0.99 a side, but drops rapidly for volume. $0.29 a side at 2,001 sides, $0.19 a side at 5,001 sides and $0.07 a side after 10,001 sides.
20,000 sides...