There is a good discussion thread on these neat items over at predecimal.com. They usually sell between $20 and $40 US dollars if (and only if) they are in good shape like the ones on this page. I see them as high as $100 in international auctions, but that is an exception. Dealers will buy them for roughly one-half retail price.
The history in interesting. These models were a suggested design by Joseph Moore and made around 1850 to replace the heavy coinage then in circulation. Some were made of two metals (bimetallic) with the inner part (supposedly) in silver to compensate for the small value of copper at the time. There is a variety with PENNY reading PENNEY and this is rare.