I guess my nights are so cheap because I pretty much only drink or go out at my rowing club, where beer's 1,20. So assuming everyone gives an equal amount of rounds end you end up paying give or take as much as you'd have paid had you bought all your own drinks, you wouldn't even get to 20 a night.
Yeah my nights out in leiden were never more than 20 euros. Tbf last night was only about a tenner. Found two quid on the floor so that was a free double. Wonderful.