First off, I have heard the cover the plate rule, but how the heck does anyone know how much that is?? Are you supposed to ask the couple? And second off, as we would discuss at my work (stationery/invitation boutique), it is an invitation to a party thrown by the couple (or their parents), so why is it expected the guest should cover the cost of it? Keep in mind, we were very picky on etiquette at our job, but it raised our hackles to hear a bride and her entourage talking about who they were going to invite, and someone would say "Oh, you don't want X, they don't give enough cash/big enough gift." My understanding is, the couple is inviting the people they love and want to share this special day with, but if they are inviting people solely to get cash, why send invitations? Go to a JP and then send out invoices! And maybe Joe (who just got laid off and has 2 kids) can only afford $25, but great aunt Ethel gives $500, so, as my one friend said when I was helping her address her thank you's, every thing is appreciated because that person came, celebrated, and had fun.
Also, and this may be regional (I am in Pennsylvania) but I have been to some really wonderful wedding receptions with spectacular food and drinks, and no one ever paid $100-$200 a head! My step brother's beautiful wedding 3 years ago had an incredible buffet with 4 meats, carving stations, 6 different sides and an open bar and it still was no where near that! I think the most I have ever heard was one of my closest friends whose was $75 a head due to it being sit down (buffet would have been cheaper but her MIL, who wasn't spending a penny would NOT hear of a buffet).