Congo isn't a rare game because they were being destroyed for MM's. Congo (and NBA Fastbreak) are rare because before replacement parts were plentiful, those were the two go to games to steal the parts from as those games were considered the worst of that period.
I'm using the word rare for consistency, but Congo isn't really rare. They pop up all the time, usually in good shape, and can be found easily.
I'd guess less than 20 were destroyed for MM, probably less than 10 especially considering NBA Fastbreak was also a candidate and worth less.
Quoted from TigerLaw:
I wonder how many Congos were destroyed in 2012 and 2013 to make MM's. A more rare game today than just a few years ago when they were being canabilized for MM parts. As others have said the price is high with a faded cabinet but it's his right to ask what he wants.