I bought my MX-6B for $750 US on eBay.   

  I was the only bidder. 
It was runnnig, complete and in great shape!  I just freshened up things (new air cleaner, new tires and new plastics).  After I raced it twice I replaced the bars, grips and clutch lever.  The original Ohlins were in great shape, but needed to be re-valved and add the proper springs for my weight.  After looking at the cost of restoring them I was getting close to the cost of a new set of modern Ohlins, so I bit the bullet.
Still my best buys have been on Craig's List.  I purchased three 1979 KX250's for a total of $1200 US.  All of them complete and two of the three are running bikes. 
 
   
  