What is average for advertising sales commission in the US?

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I just interviewed for a job today within the company I am already working for in advertising sales. I have no doubts that I can do the job because I am very confident in the product and it's a nich market for our area. My only concern is the commission they are offering. Right now they are only offering a 5% commission plus a descent base salary. The commission just seems really low to me. They did tell me that after 6 months of being employed if my sales are up I would get a good raise in my commission. What do you all think? Does this sound fair? And for anyone who's in advertising sales what commission do you make if you don't mind me asking?