In practice the commissions offered in MLS (the multiple listing service) to cooperating agents seem to run between 2% and 3% with the listing agency taking a similar amount (but not necessarily). Again, the overall commission can vary depending upon how desirable or expensive a house is, the current state of the market,
the seller's "motivation" etc.
Usually, the seller pays the commission, which is then split--generally (though not always) 50/50 between the listing agent and the buyer's agent. Remember, though, that--depending on the agreement each agent has with his/her broker--that portion might again be shared.
For example, hypothetically, say the commission is 6%. (But in reality it can be higher or lower; it's negotiable.) And say the property sells for $500,000. In that case, the total commission would be $30,000. The first split might be $15,000 to the selling agent and $15,000 to the buyer's agent. Then, if there's a 50/50 split with each agent and broker, you'd have $7,500 going to the selling agent, $7,500 going to the selling agent's broker, $7,500 going to the buyer's agent, and $7,500 going to the buyer's agent's broker.
As a home buyer, you have no control over any agreement the seller has made with his/her agent on what commission is to be paid, and how that commission is to be divided. That's all under the control of the seller and the seller's agent.
Hope that helps.