Didn't read all the responses but here's my 2 cents. Btw, to bad the OP couldn't add everyone's 2 cents to their gift, then they wouldnt have a problem! Lol!
Just came back from a wedding in NJ. We gifted $200 for the 2 of us. While we are very comfortable, we are not flush with money either. Our personal limit is between $150-200 per couple. The groom was in our wedding party when we got married so we gave at the high end of our comfort zone.
When we got married in NYC in 1996, the $$ per person was over $50/pp for a Chinese Banquet. That was the bare minimum (it doesn't get any cheaper than chinese

) - ANYTHING remotely Western would have been lots more and totally out of our budget. Of course it had to be in the city, many of our guests and family were nNYers that needed to use public transportation because didn't have cars. If anything has changed since then, it's prices have gone up.
We, personally, do try to "cover our plate" so to speak but again within our comfort zone. If the wedding reception is a "basic" Chinese Banquet in Chinatown, it's usually at the lower end of our range (been there, done that a gazillion times). If it's something different, in a different venue, etc. we do give more. I know it sounds HORRIBLE

like we are "paying" the bride and groom not to have a boring wedding! But that's just us. We know that wedding receptions outside the traditional Chinatown banquets WILL cost more no matter.
But if the bride and groom go all out and 5 star extravaganza, we don't feel like it is our responsibility to "cover" our plate either.
That all being said, how much you give totally depends on YOUR comfort zone. If $100 is all you feel comfortable giving, then don't push yourself into something you are uncomfortable with. If your DDs make you feel cheap or embarrassed and more $$ will be a hardship, my suggestion would be to enclose it with a small inexpensive gift/poem/whatever.
When we got married, we received a lot of money as well as gifts. Some people gave lots, some not. And to be honest, we appreciated the small as well as the big. We would just happy they came. We had a lot of clergy at our wedding. They along with many other people, literally had multiple weddings to go to every week!

Can you imagine if they had to give $$$ at every wedding!!! NO way.
One of my absolute FAVORITE wedding gifts did not involve money or even an expensive gift. A sweet couple took lots of pictures during my wedding ceremony and inbetween the ceremony and the wedding banquet got the pictures developed at a one hour place. They cut out a lot of the pictures and made an instant wedding collogue (sp?). They photocopied the collogue through a color copier and had it laminated for me. It was the most wonderful gift - how moving to see my wedding picture so fast!
THEY PRESENTED THE COLLOGUE TO ME AT THE BANQUET!!
While the "norm" may be a certain amount, nobody will expect eveyone to be in the "norm." Do what is comfortable for you and enjoy the ocassion.