The production started in 1936. Knuckeheads engines were replaced by the Panheads manufactured from 1948. I've made some market research and if per say you were planning to buy one of Khuckleheads you would pay depending on many factors but mostly on condition from $3000 to $80000.
It doesn't take a lot to make a Chopper or a Bobber from the bike as it already has almost all it needs but I would leave it as it is (restore if needed though) because this model is truly great.
If you'd like to know more about the history of Harley Davidson motorcycles check out this.