The key is just how risky a trader are you. If, as a trader, you are really not "risky" because you are consistently successful at managing risk (Net positive month after month), then I would imagine that the "hegde from commissions" that a broker uses against his capital at risk does not need to be so steep. He will still make a several thousand $'s a month, never really getting nervous about his exposure. A few months of this and broker is covered.
I understand that I don't put any risk cap up. I am not trying to keep it all, just something fair. My volume is 200K/mo SO I guess a 25% take is fair in this situation, as opposed to a 67% take if I was to put some money up. Hmm, equating to a $2400 difference per month, I guess it's a nobrainer.