The rough guide that someone suggested about what a meal and a few drinks in a restaurant would cost you on a night out, for two, is probably a fair way at looking at it.
You can only assume the bride and groom aren't looking at covering their costs, however, there's probably not too many places that offer a decent 3 course meal, grog and entertainment for $40 per person either. I'd be inclined to give $100 minimum, and even then that would work out to be a cheap night out if using that as a guide.
Personally, I'd rather pay a little extra than have people remember me as a tight a*se though.