Most commercial brokers do not use a rule of thumb. There will be sellers closing costs that were negotiated in the offer plus what ever commission that was agreed upon. Most of us here at Trulia are residential Realtors. Talk with a local commercial broker.
Talk to a Commercial Broker, Commercial Real Estate goes along the same principles as residential with some differences. Commissions are generally along the lines of typical commissions in residential. There is no general commission base, as they are negotiable. Best bet call and interview Brokers in your Area.