The organizations that do studies like this are the state and national nursing associations, maybe the state and national hospital associations and some of the nursing journals do polls. You can do a search on the websites of these organizations for the subject of "salary studies" by occupation or degree. It's hard to access what the nursing journals have printed without paying for it--better to go to a library to read their stuff on it.
Understand that there is a difference between the collegiate degree that someone has achieved and the job they are hired to do. A staff RN is still a staff RN in most places whether they have a diploma, AA degree or bachelor's degree. With my BSN about the only place I made more money because of it was when I worked for the government. Otherwise I made the same as an AA degree staff RN, but I generally got promoted to management and supervision positions more readily over AA nurses.