The question is reasonable, the answer is complicated. By law, if I tell you "the average commission is X%" I'm violating the Sherman Anti-Trust Act. Some agents out of ignorance will answer broadly in a forum like this, and I've never heard of anyone getting in trouble for it, but I don't want to be the first.
The best answer is to start by seeing which agents are active and successful selling land in the area. Interview a few and ask about their services and their record of success, not their commission. If they volunteer what they charge, you have your answer. If after hearing what they provide you further inquire what they charge, that's fine too.
Here's the secret, not all higher commission agents/brokers are worth what they charge (the discount agent's love this) and not all discount agents are a bargain. If higher priced agents all did a good job, there may not be any discount agents. When a higher priced agent does the minimum (or less) in the hope of an easy check, they make all agents look bad.
Discount agents however must do a much higher volume to make a living however. Do you want to be one of a handful of clients the agent is working with, or one of hundreds?
I realize this is way beyond what you asked, so I apologize. My final words are find the best agent available. A great agent/broker is worth more than their commission, but a bad one will cost you more than just a commission. Best of luck.