It really depends on the realtor. Some may charge a flat fee, others may charge a percentage of the sale price. I would recommend asking around or doing some research online to find out what the average commission rate is in your area.
Keep in mind that you may be able to negotiate a lower commission rate if you're working with a particular realtor. And, if you're using a broker, they will typically take a commission cut as well. So it's important to factor all of this into your negotiations.
In the end, it's important to remember that you're paying for the realtor's time and expertise, so be sure to get an estimate of how much work they'll be putting in upfront.