From my prior post; the floor traded $10 Dow with 5,000 vol per day and over 35,000 OI has to be eliminated to get decent vol in the DD. The presence of the $10 hurts the DD.
I was observing the CBOT live book for 5or 10min. It looks like just market makers,no public. The spread is usually 3pts, sometimes 2 pts. So whatever the savings on comm. you loose on the spread. I think they have to cut out the $10 Dow and then make a big promo push forDD.