I think one has to distinguish between a "collection of old bikes as a hobby" and "collectable bikes". The former is what I believe most forum members are doing - collecting the bikes that they want for whatever reason (but not as an investment), and in that case values cannot be predicted because a bike is worth what the buyer wants to pay. Typically that is a wide range but rarely in the investment category. In the latter case, I believe that bicycles have not yet gotten to the point of say old motorcycles or cars where they have shown the returns necessary to become "collectable" (thank heavens).

BTW - flipping bikes is not in my mind an investment activity - it may be a good way to fund the hobby and in extreme cases may provide enough income to allow one not to work at other jobs, but is not in itself a way of generating a predictable return on your seed money.