Culturally we generally give money. We rarely go to a wedding were there are gifts.

You always give a card, money inside, because that is the gift from you. The main idea is you are helping the couple with a good start to married life.

As far as amount, that depend on what you can afford and want to give. We follow the rule of thumb of cph+some. Of course that is just works for us.

And like Tasha said, normally the notice you get for the wedding helps to put money aside.