Just my two cents, but to me the obvious break points would be anything that requires special approval from AMA. AMA Large Model Airplane program (AMA Document 520-A), helicopters operated under the provisions of AMA Document #555 (metal blades),officially designated AMA Air Show Teams (AST) operating the special devices (AMA Document #718), turbine aircraft (AMA Document #510-A).
The premise for asking these groups to pay more is a simple one, if they're no more risk than other aircraft, why the need for special approval to fly/operate any of these?