Sorry Jamphone - we can't just decide bets because 'someone saw it'. We didn't see it; and there's no other evidence presented that it did in fact happen..
Just to be clear; it's possible that it did in fact happen, but without valid proof, the next person will say 'No it didn't happen'. Who's right then?
So therefore - you should annul the bet. Otherwise it is unfair. The creator did not choose a metric with historical data. What is the other option? By allowing it to continue - and on Dec 31 going "hey, nobody saw it cross $500" - you've already told me you've decided it didn't happen.