In relation to overspend HS2 is apparently going to run £50 billiion over initial estimates - and that's just a train.
In all honesty in space terms $3 billiion isn't that much. Google says that NASA's budget this year is $25 billiion.
I think Musk will get a workable - reusable - rocket that can get to the Moon and back, and whatever the cost it will have been worth it, as it will open many doors in the future.
Talk of Mars is (and was) just crazy, and I think it's stuff like this that gets people accusing him of being a snakeoil salesman. He's also done a questionnable number of other things, but that's who he is, like him or not.
I'm not a fan of Musk, but one good thing I will say is that he's at least put space travel on the table, and hasxpeople thinking and talking about it. And the more it is in the public conciousness, the more likely it is to come about. I wonder how many other countries around the world are promoting their space programmes further in fear of being left behind? Would the USA have landed a man in the Moon in the 1960s if it hadn't been for the successes of the Soviet space programme?