Was discussing this at work the other day and I had two people tell me that their belief is that you have to cover the cost of your seat at the wedding plus some. So if it was $100 per head as a couple you would give at a minimum $200. I could not believe that and have never practised that myself. I usually work out how much to give based on how close I am to the couple not how much the meal cost. TBH I would really have no idea how much the meal would cost.