Due to anti-trust laws, realtors cannot discuss commissions in public (for example, an online forum). Each brokerage is different. The only time a buyer would pay a commission is if the seller, an FSBO (for sale by owner) will not pay the commission, then the buyer would have to compensate the agent representing you.
The seller sets the commission with his/her agent--there are no standard set fees as all commissions are always negotiable. If you are a buyer, your agent will be paid from the seller at closing--unless you have a separate agreement with your agent to pay an additional commission.