The brokerage fee (commission) that you negotiate with the lisitng agent is part of the listing agreement contract. The wording in the contract explains what portion is to be kept by the listing brokerage and what portion is to be paid to the selliing brokerage.
The percentage figure is the total you will pay out of the proceeds of the sale. Remember that it is the buyer's money that you are using to pay the fee. There is a common misconception that it's the seller alone that pays the commission evne though it is the buyer that brings all the money to the table.
Somewhere between 6% and 5% is the average commission in the Bay Area, but as I said, all commissions are negotiable.
e.g. $20MM home may be reasonable to expect commission: ($2MM x 0.5) + ($18MM x 0.005) = $190K.
Of course you always get what you pay for, so again negotiate the best deal with the best broker/agent.
One of the factors that is misunderstood in residential real estate is that when you put your home on the market you are entering a competition with others trying to sell their homes. The real estate market appears different than other markets, like the stock market, because there are so many variables that except in tract developments every home is different.
Your Realtor's job is to expose the value in your home to the market. Two different Realtors can market different features of the same home are create two different pictures of the home's value to buyers. So when interviewing agents it is important to hire the agent that you think understands what your home brings to the market.
I marketed a home once and the sellers said that the home was larger than the County Assessor said, so to verify we had it professionally measured. at $400 a square foot, we just increased the value by $80,000, that persuaded the sellers that I was the Realtor best suited to sell their home.
So not all Realtors are created equal. Interview three and compare, then select the best one. So it is not how much commission they charge, it is how much money you put into your pocket at the close of escrow.
5% is the going rate for most all properties these days. When I first started 19 years ago, it was generally 6%. All commissions are negotiable and if you have an agent who is helping you buy and sell, you should negotiate a deal on that agent's commission. As with Eric, I think you may have missplaced a percentage point when factoring the commission.