And I've also always worked on the basis of your gift/cash should be same as the cost of your seat at the reception.
That's what I've always been told was the "right way" to work out how much to give I guess it might differ based on how close you are to the couple. Also, if you had to travel interstate or out of the country then I would almost assume that THAT is your gift to them. I certainly wouldn't expect a gift or money if I had asked people to pay to travel.