I'm sitting here trying to figure out whether I should take a fence or just go away OK, back to work...
I suppose you have to pay scaled, volume dependant commissions. As usual in the industry, larger volume should result in lower commissions, or?
The chief advantage MM have is that they buy at the bid and sell at the ask. Because they stay delta neutral at the end of each day they are permitted much higher margin ratios, which makes it feasible for them to buy and sell large blocks of stocks or futures for delta hedging.
CBOE makes a revenue per contract (RPC) of between .17 to .5/contract so they must be charging brokers on top of this. Really good contracts from IB etc... will get you close to what the broker already has to pay CBOE. Of course, this is assuming tight bid/ask spreads. You can see the the RPC information on the CBOE Investor presentation: http://www.scribd.com/doc/50460557/CBOE-Investor-Presentation-Raymond-James-2011-final-3-4-11