"$500,000" is not the cost for a ticket on the first mission, it's the eventual cost in the steady-state.

There are a lot of pieces left that need money dumped into them. Musk seems to be building all the technology pieces needed for a Zubrin "Case For Mars" trip. [1] Not all of those have immediate market needs that SpaceX can justify, such as artificial gravity. (NASA is extremely reluctant to have anything to do with this. Their tune might change if someone else can demonstrate it safely, but for now NASA policy is to only talk about microgravity, not artificial gravity.)

SpaceX needs to stay profitable. That is extremely important, because people need to be able to build plans around the company continuing to exist. It can't have loss-leaders.

However, Musk can be a loss-leader: he can hire SpaceX to (say) research, develop, and build artificial gravity labs. SpaceX would be hard pressed to find a business reason for building those in the intermediate term.

The "Case For Mars" got a ~$30 billion price tag from the same committee that gave the 90-day report a $450 billion price tag. SpaceX has demonstrated the ability to significantly aerospace costs, but even if they cut it by a factor of 6, that's still $5 billion in development costs that a company with a total valuation of $10 billion somehow has to support. (And "selling tickets to Mars" won't give them revenue for at least a decade.)

[1] The rough list of big technologies needed is about 5 elements long: 1. Heavy lift. 2. Methane-burning rockets. 3. In-situ resource production. 4. Artificial gravity. 5. Mars EDL. SpaceX is working directly on #1 and they have a nice research project going on #2. Methane rockets are justifiable by SpaceX because they have (some) practical advantages over LH/LOX rockets.

Good points, though I don't see why artificial gravity makes the list. Not only is it already a solved problem (centrifuges are easy enough to make) but it is also not necessary. People already live in space without the benefits of gravity for longer than the trip to mars would take.

Centrifuges in space are not a solved problem. AFAIK there has been exactly 1 experiment in space with artificial g, at very low levels of g. You want to be able to maneuver and orient your ship even while it is spinning, and you want to study the failure conditions (most plans call for two tethered modules held by cable -- you want to experiment with what happens if that cable under tension breaks).

Also, we just don't know what 3 years of Martian gravity does to a human. It would be good to see just how much better 3/8 g is against 0 g.

Microgravity is hell on your skeletal and cardiovascular system. Artificial gravity is a must for interplanetary travel.