Hi all,
I think nowadays, weddings, be it lunch or dinner, both are costly. And considering the food, wine and the reception cocktails that you are having; as well as the photos taken, a min $100 is the least you could give that couple a good blessing. Traditional Restaurant or Hotels Banq are the same. Perhaps hotels $118 at least.
Also based on how close your relationship with the couple, you should give even more. As this also show the respect for them and your sincere blessings.
I had seen people who are so cheapstake as to give only $30 or $50 while attending weddings. Please do consider, that the couple even bother to invite you in the first place.
Just my honest thoughts. Cheers.