In my company we are paid an additional bonus on top of the salary as TRI/TRE. It is calculated as a certain percentage of the salary, so is linked to and increases with salary.
When we are doing conversion courses, we get an additional hourly bonus on top.
What is being suggested in your company does not sound good to me. The amount of work involved being a TRI/TRE is more than just the hours instructing or checking,