The 66 & 99 are not Featherweights. The 221 & 222 are Featherweights. The 221 is the more common model. The 222, with the freearm, are not seen as much. (I believe fewer were made in the first place). Unfortunately the prices are all over the board. I would not be willing to pay more than $200-$300 for one, personally.
*Fortunately I have mine that was given to me by my Mom & Dad when I was 12 years old :)