If considering M3's meant to be used daily and not ones that are deemed 'special' in any way, I would consider 1957 as transitional starting around 854xxx. I've never put my hands on a better M3 than early '57's. I personally would value user pre 1957 M3 a little less than post 1957, but this can be modified by cosmetics, ownership and maintenance history. The above one million M3's seem to fetch slightly higher prices, although I I'm not sure why exactly.