hmmm, sounds crazy, but wouldn't there be a chance to catch the peak with a meter that has a peak hold or so. That could at least show in which ranges the voltage can be (if the peak-hold is fast enough). Well, meant as question for all around...out of curiosity.
I would say that it makes a difference on your circuit..... what voltage or current is going through your coil, is it just the coil shut down or is there some capacitor or so to slow the shut-off-time down?
EDIT:
http://www.physics.unlv.edu/~bill/PHYS483/relay.pdf Here they give 20kV as value, but that's in automotive use and you have high current running through the relais. Therefore it could be seen like an old "contact driven ignition" system which just is just switching an contact to establish and break a current flow from the battery through the ignition coil (which is a transformer here).
So I expect your voltage is lower, but don't know....as we don't know your circuit.