My policy always was, my pager compensation was proportional to the potential impairment of my own agenda.
What I mean is, if you expect me to reply to you within a certain time frame, then I have to be near a phone or within cell coverage. This restricts where I can go. If you expect me to connect in remotely, I have to be near internet connectivity, and most of the time be carrying my laptop with me. This further restricts where I can go, and what I can do when I go there. If you want me to be on site within a certain time frame, that even further restricts where I can go.
If I can watch TV, go to the movies, or out for dinner and still be on call, that's not going to cost you as much as if I have to be within 30 minutes of being on-site from the moment you call me.
Historically, I have been lucky. One employer paid us $500/week to carry the pager with a 90-minute call-back SLA (and then hilariously lost the pager number and refused to admit it, so was unable to call us for 8 months). One customer was quoted something stupid like $5K/week for 7x24, 60-minute on-site (plus hourly when we got there). Any call time was billed back to the client, and we (theoretically) got time-for-time in exchange for that. My current employer has a pager our customers to call, but since it is 7x24 it is optional to be in the rotation and for various reasons I've opted out. In addition to receiving money for your week on the pager here, time is tracked very strictly and we get time-for-time for any pager-call time served.