There's been a few on ebay advertised as "rare". If I remember right,there was about 2500 1977 Hondamatics sold in the US. I don't know if this makes them rare,but there's not alot running around. But,there's not much demand for them. If the one you're looking at runs and doesn't look too bad (rust) it might be worth about $400-500. At first glance,it looks like many of the parts might interchange,but many of them are different. The frame is different-you can pull the head without taking the engine out. Things to check:Front brake hose,carb to intake boots,idle speed diaphragm,parking brake-none of these are available new and are almost impossible to find. If it's cheap enough,buy it. It's quite the conversation piece on bike night.
........just my thoughts, Paul