This is all very regional and dependent on your circle. Our friends in the northeast use the registry for bridal shower gifts and then give cash at the wedding. Here in GA and when we lived in FL, people used the registry for the wedding itself.
We've given a wide range of cash gifts over the years, depending on how close we are to the couple and whether they were friends from the northeast or here in the south. We have declined invitations from acquaintances and pick a $50-$75 gift from the registry to send. That's also my typical price range for bridal showers.
My husband was sure we were going to get mostly checks at our Atlanta wedding even after I assured him that was not how it is done in the south. He is still shocked that we had a car full or presents to bring back to Nova from Atlanta.