Compensate for 24×7(after hours)support?

We want to move to a 24×7 after hours IT support environment. However, rotation may not be a good fit for us since each person is tied to a particular function or application and the end users requiring the support are in different states around the country. We are looking for information regarding wages for our tech support personnel that will be answering pages, cell phones, and/or coming in to the office to support end users after hours and on weekends. We have heard of differentials but unsure what may be the range for the differential pay.