If it's your wedding: R0
If it's your mother's wedding: R0
If it's your son's wedding: R100
If it's your daughter's wedding: R100 000
If it's your friend's wedding: Don't give cash.
If it's a relative in the 2nd degree: Don't give cash
Give what you can afford.
I seem to remember something about the gift/cash should at least be the same price as the meal, but that could just be me making stuff up.