Isn't half of that 400 million a kind of retainer to keep ULA available? I thought they're still paying that, so it's not much cheaper yet. And they might still need to pay it to someone.
I think the point is that the 400-to-100 figure is not Apples to Apples. Imagine 20 years from now SpaceX' ITS has failed to finance and their Falcon Heavy is a loss leader because too few payloads need it and the rest has moved on to a new rocket. This is like the USGov paying them a few billions a year to keep Falcon Heavy around because it's the only rocket with some specific capabilities. IMO depending on the specifics this is not so unreasonable. It's still great that SpaceX has achieved a factor of 2 improvement already for the base price compared to ULA, which doesn't yet include reusability, but one shouldn't overstate how much ahead they are.
Whatever investment the US government have given to SpaceX, they'll get it back very quickly now that competition exists in the launch market.