Screw this...it isn't a sliding scale...
I can recall to this day families (that I couldn't give a flying F) if they were invited to my wedding...that my wife included...
Family of 5
Cost to feed: 125 dollars (not counting what they scarfed down during the open bar)
Cost of gift from them: 15 dollar blender from Walmart that we later found out was SO damn old it wasn't even in their system...it was an ancient re-gift...
Basically tossed it in the trash because it was a POS
My advice would be to cancel the ceremony and go to Vegas...have a great time and have a small reception later for those you care able...save big time!
My wife and I went to a wedding last September and I couldn't believe the trash that showed up. There was this family of white trash rednecks that showed up wearing shorts, t-shirts, camo, etc. and they were loud and rude. So when the reception came, we were at table #6 and they were at table #9; the number of the table indicated when you got to go up to the buffet. The white trash dad came over and switched numbers with us. I thought he was doing it to be funny, but he kept the numbers switched.
Had it not been a wedding, I probably would've put my foot in his ass. The moral of the story, folks, is that there are three types of people:
1. People who hate weddings
2. People who go for a free meal
3. The people getting married