A lot of it is psychological, 0 to .0002 per share is out there, but there will almost certainly be some variation of a profit split, misc monthly fees (desk charge, administrative fee etc) or in some cases additional or marked up transaction costs.
But all firms whether they are broker dealers or other forms of prop shops are a business, there is an overall cost to doing business and firms will find a way to make it or go out of business. If you expect any kind of service from the firm that you are with then sometimes paying a competitive but still normal rate is worth it.