Coil output voltage is: Whatever it takes to jump the gap. Up to 40-50,000 volts, that is.
With a light load on the engine you'll see secondary ignition voltages roundabout 5,000 volts, depending. A throttle snap will get a spike up to about 12-15,000.
An open circuit (bad wire, plug, etc.,) will show voltages over 20,000, unless there is a short to ground that keeps it lower, in which case the waveform will look abnormal and the problem found that way.
"Silly-scopes" can tell you a lot once you know how to read them (lotsa practice), too bad they're so damn expensive.