The other thing to check that affects value is the condition of the fuel tank on the inside....A rusted/gummed up tank is relatively useless, finding another Ducktwin tank in good shape would be difficult. It is only fair to assume that any engine you buy will need ignition, fuel system, and water pump servicing. I would have a closer look at this engine, checking compression and looking for cracked/bent castings, inspect the tip of the skeg also to make sure it is not snapped off. A relatively low hour engine in good basic mechanical condition (good tank, good compression, no overheats, no bent/cracked castings) that is in nice mechanical condition is worth about $100 I'd say.
But, like everyone says, it depends on how much you want one of the rarer Ducktwin models. I have certainly spent more than $100 on engines I really wanted.