It is absolutely what you can afford. We generally give $150-$300. For my sister, i made the wedding cake and paid around $200 for a suprise hens. We also spent the night at mine prior to the wedding so i provided dinner ans breakfast etc for the bridesmaids. This meant we gave $150, which i considered to be from my husband as i had done enough!
We had a wishing well and received between 0-$1000. Everyone gave a card and we also got some gifts of "time". Our hair was done as a present by a hairdresser friend, and a tradie friend did some work on our house as a gift. I consider time so valuable so appreciated these the most!