How much does a Japanese giant salamander weigh?

Of course it depends on the particular salamander. From Wikipedia: "They range in size from the minute salamanders, with a total length of 2.7 centimetres (1.1 in), including the tail, to the Chinese giant salamander which reaches 1.8 metres (5.9 ft) and weighs up to 65 kg (140 lb). Most, however, are between 10 centimetres (3.9 in) and 20 centimetres (7.9 in) in length."

So, in grams, that's anywhere from less than 1 gram to 65,000 grams.