I went to a lot of weddings when I lived in Michigan, a lot of "ethnic" ones. The custom up there was "cover your plate", which meant to give a gift, monetary or otherwise, at least as expensive as what the cost was for the guest to eat at the reception. I'm not sure how you were to know how much the cost was. Most people don't know in advance whether it is going to be a filet-mignon-and-lobster reception or a rubber-chicken-and-pasta reception, so it's hard to figure that out unless you just come out and ask.
Another custom up there was the "Dollar Dance", where you get to dance with the bride and then you pin money on her veil. I did see more than one bride with a pretty little white purse hanging on her wrist during the reception.