It's not as expensive as it looks, Starship plus booster costs around 100 million. A Saturn V Apollo mission cost 185 million in 1969 which, according to https://en.wikipedia.org/wiki/Saturn_V#Cost, would now be a bit less than a billion dollars.
Also, SpaceX is not building rockets, they are building a rocket factory. If they succeed they will have lowered the cost of putting stuff into space by an order of magnitude. The potential rewards are huge.
Yes but at this point the upper stage is barely a spaceship. Mostly an empty shell. And they have spent $10 billion so far on something that barely flies.
R&D and prototyping is an up-front expense. Amortization over many units spreads out the costs to long term profitability. Does SpaceX have that kind of time, though? A prospective global depression would dry up the capital for funding Starship development.
Also, SpaceX is not building rockets, they are building a rocket factory. If they succeed they will have lowered the cost of putting stuff into space by an order of magnitude. The potential rewards are huge.