For one, I would only give money at a wedding. Gifts are for the bridal shower. Second, I think $100 per person is the norm, so if it's you and a guest, $200 is appropriate. However, sometimes more (usually not less) is warranted given the relationship. I think the rule of cost of the meal comes to account as well. However, when you consider cost of travel to some weddings, sometimes I won't consider adding more because they chose a fancy spot that cost me a ton to get to.