If I take a listing to sell a home it's usually 4.5% of the selling price. If another agent brings a buyer, I split the comission (2% my rate, and 2.5% buying agent).
If I happen to have a client that is interested in that listing, I only charge 3.5% total, and rebate the 1% back to the client.
Everyone has their own fees, however an agent will not be able to negotiate too much on the commission, where as a broker will be able to negotiate aggresively on the commission (as I have done in the above example).
The typical market rate for So. Cal. is 5%, and that is split between listing agent and buying agent. Even if the same agent represents both buyer and seller, they still charge 5%.
Finally, just like any other commodity, it is a supply and demand driven cycle, and commission rates vary, based on competition.
Henry