The quick answer to your question is that there is no limit (either high or low) on the amount of commission an agent can ask for. Commissions are really a negotiation for service. Often times a percentage looks strange but is driven by the actual value of the property. For example, a 3% commission on the resale of a $4000 lot would be $120. Very few agents could afford to represent that transaction for that amount of money. In my case my cost of E&O insurance would be about that so I would be working for free. On the other hand charging the same 3% for a 4 million dollar property would earn me a commission of $120,000 for one transaction... also not a very fair commission.
I would make two suggestions. First, figure out how much actual money you are talking about for your transaction and see if it sounds reasonable. If not then you may want to negotiate for a different number, but keep in mind that getting an agent to agree to a commission that is way too small will also mean your agent can't invest the time and money he or she should in order to sell your property. Second... and this is important... your agent should be your closest ally during the sales process. If you are not comfortable with your agent at this point then you will not be able to depend on them when you really need them later in the process. Find an agent you are comfortable with. Make sure you are comfortable with that agent, then depend on them to act in your best interests.
The fact that you have posted this question about commissions suggests to me that you may need to start the process of interviewing for agents again and see if you can find someone who is a better fit for you personally. you may not find an agent who will work for less, but if you find an agent that you feel is earning whatever commission you agree on then the search was well worth the time.
Best of luck with your agent hunt, and your transaction!
Casa Grande Realty