Decades of expertise might be a reason here, France has the longest amount of (high speed rail miles / country area), and the non high-speed network is also huge. It's also possible that the monopoly of having only one, state-owned, railroad company (SNCF) let that company negotiate/choose better prices much more aggressively than if there was many companies. I don't think it was stupid at the time to have such a state-owned monopoly for building this key infrastructure.
Also I'd think that with that big of a network, some things start to become less costly because of economies of scale. Last April SNCF bought 30 trains to Alstom for 250m€, with the current US infrastructure I doubt the US needs that many trains. If one day like France they have 450 high speed trains, maybe it'll cost them less to have more built.
Japan and France are dense countries, the area of France is roughly equal to the area of Texas, it's hard to imagine 450 high speed trains there, where they're struggling to even get one high speed rail line between Houston and Dallas.