The repository linked mentions that each stage should cost about $500k and there are 4 stages. You can argue that he is not asking for it, but at least he is valuating it at $2M.
200k per year would not be the worst salary to pay a developer for this work, and I'd say he's outlined about 5 to 10 years of work. Feels like you could negotiate a package of escalating compensation contingent on delivery milestones.
