There's so much wrong here, a ton to unpack.
> A fully reusable Starship has a launch cost of around $75m - $90m
No, that's the Starship build cost, i.e. the cost of an expendable Starship. A fully reusable Starship currently does not exist, but reusable launch cost be around $5m/launch (amortized).
> the last V3 launch managed 44 tonnes of payload
Intentional, Starship wasn't fully loaded.
> on a sub-orbital flight
Intentional, test flights are sub-orbital.
> of not even 200km
Intentional, done to target the landing site in the Indian Ocean.
> That's an optimistic launch cost of $1.700/kg
You can do basic math, but you are intentionally using incorrect numbers. Garbage in, garbage out.
> I have no idea where you pulled your $400/kg number from
Starship V3 manufacturing cost (one-off, not mass manufactured) is around $80-100m. Mass manufactured V3 would be in the $50m range. Starship V3 has 100T payload capacity to orbit in reusable config, see https://www.nasaspaceflight.com/2025/05/future-starship-bloc... . In expendable config, Starship can carry 200T, see https://newspaceeconomy.ca/2026/04/16/detailed-review-of-sta...
Using today's numbers, we get:
$80 million / 200 tons = $400/kg to orbit (fully expendable).
This number is already exaggerated, the booster is already proven to be reusable.
If the current Starship is mass produced, this improves to $50 million / 200 tons = $250/kg to orbit (fully expendable).
> To be economical at all, Starship needs to reach its target capacity of 100 tonnes to orbit
You do realize the Starship + Booster stack weighs 5,000 tons, and that a 100 ton payload is only 2% of the rocket mass? And that 2% is an achievable fraction, both Falcon 9 and Falcon Heavy have a payload fraction >4%. The Starship upper stage alone weighs 1,600 tons.
> refueling requirements
In terms of problem difficulty, orbital refueling is a minor engineering challenge to solve.
> fuel boil-off in orbit
I hope you are being facetious at this point. How do you think LNG is transported around the world? You realize this problem was solved decades ago?