The way I see it, it has multiple components:
1. the pay for being on call, just for carrying the pager and being available.
2. the pay for having to go in for a case.
3. the hourly pay while at the hospital.
I have heard about #1 being as little as $200, if chances of being called in are slim. I have also seen #3 being the usual hourly pay, with #2 being an extra hour. Of course, I don't recommend these rates, more like 50% higher.
You should also consider compensation for a lost day of work if you waste your night on a case while being paid peanuts.