Thanks Kriggy! And welcome to totally different approaches!
Up to now I've looked at isobutene, propene, chlorine... because they're cheap, that's the only reason. 1M€/200t would be a reasonable selling price for the oligomerized propellane, so every reactant must be dirt-cheap or recycled (like NCS can be if I read properly).
Malonic acid would be obtained from dimethyl malonate, itself from dimethoxymethane obtained from formaldehyde and methanol, is that it? Or by "fermentation of glucose" as wiki tells? On Alibaba I see malonic acid around 7$/kg in ton amount, that is 13$/kg of isobutene. Maybe production at the plant can improve that?
I suppose AgCl from Hunsdiecker is completely recycled.
I have a small doubt about the number of remaining carbons after Hunsdiecker.