Japanese weddings require the giving of a gift of cash.
The amount must be an odd number (so as to avoid 4 - which is "shi" and also means death).
1 is also not acceptable as it is considered stingy.
I will do this in $$ to save you conversion:
If you hardly know the people: $300+
If you are great friends: anything up to $1,000
If you are family; $1,000 +.
(Funerals are similar and, similarly, the cash is given in special envelopes - one type for weddings and one for funerals - with your name on them so THEY WILL KNOW.)
The after wedding party is usually about $65-$100 per person.
The idea is that these people are starting their lives together so this helps set them on their way.
What is also means is wedding invites are scary things!!
People you rarely see will send invites as they want as many people as possible at the wedding - obviously!!!

(Footnote: boyfriends and girlfriends are not considered real partners so dont get invited to the partner's friends weddings.. no, i am not dissapointed!!!)