Generally speaking, there should be no additional fees for filing 485 when you hire an attorney for I-140, let along to say it is charged by person.
However, it may be different according to the contract between your attorney and you.
If the contract does not obligate you to pay your attorney to file 485, you'd better DIY. Most people know it is a piece of cake to file 485 by themselves.
FYI http://web.wenxuecity.com/BBSView.php?SubID=immigration&MsgID=310806