REALTORS will be hesitant to answer this, as we might be accused of "price fixing" - and that is against the law. But, you are smart to ask, because it does vary regionally; and it also varies depending on the type of service and type of property, For example, land is often a higher %, above certain price points is often a lower %; and if you are looking for something unique that might not be found in the MLS and the agent may have to go hunt for it for you, then there may be a nonrefundable up front fee in addition to a success fee. So, while your question seems like it should be able to be answered easily, keep in mind there are variables.
The prior answer came from a person in another area, I don't know if he knows NY or not - and NY is different from everywhere! The best way to find out is have a real estate agent run MLS listings for something similar to the type property that you are planning to buy and see what the "average" is that is being offered to be paid by sellers - that will tell you what is the customary fee.
I hope this is helpful