As the others say, commissions are negotiable.
If you're making the offer, that's something for you to discuss with your Realtor beforehand. It's likely the Realtor (or his/her brokerage) has a number, or a small range, for expected commissions. If you're not happy with that, negotiate or interview other agents.
In most cases, the commission would be the same, or very slightly higher, than what they'd charge for a listing on the MLS. Again, though, it's negotiable.
If you're receiving the offer, it's not your concern; the commission will be paid by the buyer.
Hope that helps.