Unfortunately, they are not worth all that much. Gun Broker and Auction Arms have them occasionally. The upper price I've seen one go for recently is a little over $300, but that is way high for most.
They're decent, workable shotguns, a step above many of the single shots that are out there, but there just doesn't seem to be much interest in single shots unless they're specialty guns from one of the big name manufacturers.
I have a Ithaca Super Single 66 in 20 gauge. Kicks me like a mule, but has never given a bit of trouble.