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.