What is a rusty '66 Formula S worth?

I found this '66 Formula S for sale, I know it looks beat up & it is very rusty plus I'm not sure it has good frame rails, it does start and appears to have the 273 Commando engine still in it. Lots of surface rust in the engine compartment and the interior is old with a lot of use. Of course I'm concerned it'll rust away to nothing by next spring but kinda like it. I'd like to make a reasonable offer but don't know what that would be (I won't even mention what he is asking). What does anyone think this is worth? Or is this a fright pig and I should just stay away from it?