* For every hour you're available for on-call you get your regular hourly rate.
* For every incident, which involves you working, you receive twice your hourly salary for the duration.
So companies based her tend to have that as standard, though I'm sure some companies would pay more to stand out.
(Or is there a 40 h/week limit or night work rules or something that prevents that?)
You have your team of 20+ developers/sysadmins working Mon-Fri, 9am-5pm, and then you have a single individual responding to on-call events outside those hours.
If you're working 7.5 hours a day, you're on-call for 16.5 hours Mon-Fri, and 24 hours for Sat/Sun. That gives a total of (( 16.55 + 48 ) 10) = €1350.
So yes, you get paid a lot. (Of course the hourly salary there was low to simplify the numbers, but you can see the income for working a week on-call is about the same as working three normal weeks.)