Money Wedding Gift

My husband and I will be attending a semi-formal day wedding for a close friend of my husband`s brother. The wedding is in the south-east. We once have received $50 from a couple at our north-east wedding who were from the same state. In our area it is usually the cost of the plate or $100 per person. How much is appropiate to give with out coming across cheap. We dont want to offend the couple but often we go overboard thinking to much of others and not of ourselves.