The answer is they vary and are fully negotiable. Unfortunately there are some real estate agents who use "scare tactics" to discredit anyone who tries to win business by "undercutting" others on pricing. Just because an agent is charging more - doesn't mean they are going to do a more effective job of marketing your property. Some agents work "harder and not smarter", then pass their less effective, but more expensive, marketing costs onto the consumer in the form of a higher commission. Interview a few agents, and ask them the specifics of their marketing strategy. Ask them "why" they are using specific marketing techniques, and also ask for hard data showing why the specific technique is effective in today's market.
The fact is - there are a lot of agents that get a listing contract, drop it off to their office - and let an assistant slap it onto the MLS and various websites and sit back waiting for phone calls. They often don't have hard data to back up exactly what sites they are on, and why. A marketing plan should be custom, and tailored specifically to you property, and the type of buyers it will attract. Your agent should have an answer for every aspect of their marketing program - and why it's included in your plan. Spending hundreds of dollars in newspaper ads, and postcards that are sent out to people who 99.9% of which aren't in the market to buy a home - simply doesn't work today. Your agent should know "how" buyers search for homes today, and be able to explain everything to you in detail.
As far as the commission goes - everything is negotiable.