What does a background performer (extra) make on average per month?
It depends on the job. The lowest you will get paid is about 15,000 yen. It should be for a short job with not much waiting. I met a girl from England at a job a few weeks back who was paid 250,000 yen for pretending to be a backpacker and pointing to a map. She said the job took all of about 35 minutes. If you get restrictions, then the pay goes up. If it is a print job also, then you get more. I would say that the minimum you should get for a job you auditioned for is around 40-50,000 yen. If you get all small jobs, then I guess you could still make 150,000 yen a month. The big jobs though are the ones to get. It is one of those things that the only way to find out is to actually do it.
