Wet nurses were not only for the wealthy, because people historically understood that feeding babies something else led to dramatically lower survival rates (in that context, obviously the same is not true for modern formula). There is always more to be said, but I have a previous answer to In older days let's say before infant formula was invented how did lower class/poor families tend to/feed newborns in circumstances where the birth mother died during labor and couldn't afford/ get a wet nurse that may be of interest to you.