The bride in the OP is a selfish gimmee-pig or an idiot or both.
First, the budget you can afford determines how many guests you invite. This is a simple rule. Why can't brides learn it?
If she's really determined to have 700 of her "nearest and dearest" (I don't even KNOW that many people!) at her wedding, she'd better be ready to lay out $1,400 MINIMUM for the cake.
Somehow I don't think she actually wants all 700 of the invitees at the wedding. In fact, it appears she's counting on it. But you'd better believe she's excpecting a pricey gift from everyone of those invitees (how dare s/he give me less than $100! It's myyyyy weddinnnnnng!).
As for any bride who requests her guests to supply any part of the wedding, up to and including "pot-luck" receptions. That is just rude rude rude rude rude. Not only would I not attend such a wedding, I wouldn't send so much as a card to the "happy couple" and I'd seriously rethink whether they are people I want in my life.