You mean a fee to pay the part time employee/PE? I don't think there would be a "standard" fee. It would be as the market demands and/or the PE demands.
Most states allow engineering advertising as long as it is not false advertising. You also would need to look into your state's requirements for engineering firms in terms of their registration with the state. Some states require principals to be licensed engineers in order to be legal in offering engineering services to the public. Others allow part time employees to be the PE (i.e. Texas).
Check with your state board on that.
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