One of my forum members, MadMichael, has a collection of antique motorcycle advertisements that appeared in magazines many years ago. These ads are pretty interesting and reflect the times they appeared. You'll find ads from WWI and WWII.
The prices were very low in those days as you'll see. There are many Harley-Davidson ads, a few Indian ads and then some ads for obscure makes. Take a look at my Antique Motorcycle Ads Gallery.