They are a novelty that is in decent demand, and appears to be gaining popularity. I believe there are three versions, and early ones can sell quite high ($400+) when in good shape. If you don't have the lower spool holder (case) they can sell for as much as the machine - IF you find one.
I've paid $150 for the first and more recently $200 for an older one, both in treadle cabinets that I could pick up myself. I've seen heads sell for quite a bit more on eBay, and ones in cases (with motors) in nice shape can go even higher - if a collector is in the market. But then you have to have exactly what they are looking for, and not something that only "looks like" the rarer ones.