Wedding Gift Amount in Midwest

My husband and I are going to a wedding next weekend. It will be our first wedding in awhile and we feel out of touch with the amount to spend on a gift. We also seem to disagree on the amount. It's a long story, but this is the wedding of his younger 1/2 brother, with whom he has just developed a relationship within the last few years. They didn't even know each other until they were both adults. They live in different states so don't see each other often and aren't close, but do seem to have a lot in common and communicate by email and holiday letters, etc. We already sent a gift valued at a little over $100. I thought that was a decent gift. My husband feels like we were too cheap and should supplement the gift. We've already recieved a thank you note from the groom so they would have to write another note if we suplement the gift. Also, we are spending a lot of money for our entire family to travel out of state for the wedding. I think that should be a factor, but my husband doesn't. We live in St. Paul and the wedding is in Indiana. Anyone know what the current going rate is for a wedding gift in the Midwest? Was our gift sufficient or should we supplement it?

Edited: Sorry for any confusion. The $100 gift was a wedding gift selected from their registry and sent early. We were not included in any showers or pre-wedding parties, except the rehearsal dinner, which we are attending the night before the wedding. My husband will be golfing with the groom and some other relatives a few days prior to the wedding as bachelor party sort of thing.