First, it's negotiable.
Second, different commissions may be charged based on a whole series of factors: The amount of effort/marketing you expect your agent to do. The price of the home. The business model used by the agent and by the agent's broker. Whether the seller wishes to buy a home using the same agent. Customs and practices in different areas of the country. And the list goes on.
Call half a dozen agents in your area. Ask them what they charge. And make sure you ask what services you'll receive for that commission.
Hope that helps.