Personally I'd give nothing, maybe a card at a push especially if you feel you were an add on guest to make up numbers. I got invited once to one of my bosses son's wedding, I couldn't have picked him out of a line up and really think we all only got invites out of politeness or numbers who knows, either way there was no way any of us were actually going as it was 4 hrs away and realistically the people we would know would be the boss and wife and ourselves! I didn't give anything other than return the card but one of my colleagues felt she had to give 50 quid voucher at the time, bit annoying as none of the rest of us were doing that!
Anyway there is a lot of asking people who you know won't go in the expectations of getting some sort of gift, I was involved in wedding business for good few years and you'd surprised at the stuff that goes on. That said my own brother invited 400 to his wedding, now in fairness only about 280 showed up but his theory was that once they covered the cost of their meal with a gift (back in Tiger days when you'd get a nice old cash gift) then he wasn't out of pocket as still had to pay same amount for band/flowers/dress/car etc, fixed costs as such, only variable was the meal numbers.