The NRT allowance is JPY15500 per day.
Some 24hr slips get an extra dinner allowance (JPY9150) because of the flight timings, but this also happens at other ports such as SIN as it is part of the allowance agreement.
The NRT allowance is protected at the above rate as this was part of the agreement when the company changed the accomodation located in Narita instead of Tokyo. (there is no bonus or extra allowance, it is just based on Tokyo 10 years ago and will not rise until Narita hotel meal prices exceed this amount)