The shift differential you get depends on how badly the facility needs the staff. Some places have it some don't. With the nursing shortage worsening, I'd expect to see more places offering it.
For fun, you could sign up with a local nursing or travel agency and find out what rates are being offered in your area.
Many staff nurses rather than working overtime will sign up with an agency for the higher rates and separate paycheck. Long as benefits are covered it might be worth your while.
Andrew Lopez, RN
Nurses' Views of The Nursing Profession
http://www.nursefriendly.com/views