It was on the order of $500k - he used solar to crack hydrogen, then used the hydrogen to make electricity.
It worked - he powered his house.
Yeah, that's about the cost I recalled (and have found online). What I haven't found was his estimate for what one of his "commercialized" systems would run, which was "much" less. I seem to recall that it ran around $50K, but I certainly wouldn't swear to it.