Has he said what he is going to do and for how much? Cleaning the carbs is something pretty much anyone can do at home with some basic tools. If there is rust inside the tank, then you would want to drain the tank completely and then go through and kreem the inside of the tank (or what ever similar product you have in Japan).

Look up "Kreem tank liner" to get an idea what your looking for and how to do it.

If you have rust damage that is all the way through the tank, then it would require some extensive repairs that may be beyond the standard shade tree mechanic.