If you are invited to a Japanese wedding ceremony, you will have to give the couple some money as a congratulatory gift. There is a rule regarding the amount. The first number of the money has to be odd, because if it is even, it can be separated, which implies the couple may get a divorce. So giving 30,000 yen is common. 10,000 yen might be a bit low. (As the economy in Japan is not so good, some people might give 20,000 yen.)
In the past, my sister was invited to a wedding party three times in a month. So she had to give 30,000 yen (90,000 yen in total) to each couple. If it happened to me, I would go bankrupt. I’m lucky to have a few friends.
I heard some people pay to arrange guests as their “friends”. Also, I heard that some people organize fake wedding parties in order to get money.