Agent Comission - Selling numerous properties with the same agent

[Palmer Thomas]

I'm going to be selling numerous houses over the next two years and am planning on using the same agent for all the houses.  I also purchased all the houses using the same agent.  The total sale value of the houses will be in the $2.5 million range.  At a 6% commission that would work out to $150,000 total or $75,000 for my agent alone.  This seems excessive to me.  

Does anyone have any suggestions for negotiating a better commission rate?