“three private citizens who paid $55 million each”
Folks the point you are missing is SpaceX just collected $165 million dollars from three guys that have a lot of money.
How much does it cost to get this rocket there and back?
How much PROFIT did SpaceX just make?
How many more billionaires want to do this?
SpaceX is making money sending satelites into orbit.
IF every time they send a rocket to the International Space station, they have 1-3 paying customers along for the ride , we are talking a very profitable enterprise.
We also have more specific figures available because Elon Musk revealed the marginal cost of launching a reused Falcon 9 in a May 2020 interview for Aviation Week (starting at 17:53). Marginal costs represent only the costs resulting from relaunching the Falcon 9 after its first mission is already done and paid for. According to Elon Musk, the marginal cost for a reused Falcon 9 launch is only about $15 million. He explained that the majority of this amount was represented by the $10 million it costs to manufacture a new upper stage. It is not reusable (and never will be), so it is necessary to make a new one for each launch. The remaining $5 million include costs of reusing the payload fairings (Musk probably only counts fairing refurbishment costs in this scenario because it costs $5–6 million to manufacture a new set of fairings), helium, fuel and oxygen, and also the cost of recovering the booster and fairings. Most importantly, the cost of refurbishing the recovered booster is only $250,000, according to Musk. That's a very low amount, which could indicate that the booster refurbishment process does not require much manpower, expensive hardware replacements or complex inspections.
https://www.elonx.net/how-much-does-it-cost-to-launch-a-reused-falcon-9-elon-musk-explains-why-reusability-is-worth-it/
How much PROFIT did SpaceX just make?
Quite a bit.
How many more billionaires want to do this?
Over 2,000 billionaires, not to mention thousands more who are nearly billionaires. You only need to be worth about $200/$300 million to be able to afford this.
As far as I can see, the only limitation right now is that the Space Station can only accommodate up to 7 people right now. And most of those slots are taken by governments and big corporations
Of course Musk could build his own Space Station soon. I wouldn't put it past him.