I never understood why weddings were so darn expensive until I became a bride myself. It's not the little miscellaneous decorations that costs so much, it's the costs per person. If you're on a tight budget, you may want to re-think about inviting that friend you haven't talked for ages.
Below, I have done a VERY rough estimated average cost of how much it costs to have a guest come. Obviously not everyone is the same, but this is what I have typically found with doing some research and shopping around.
That's why "typically" people give $100 per person for the gift to the bride and groom.