Propellant contributes very little to current launch expenses. Vehicle and infrastructure costs dwarf it. For example, it costs SpaceX less than $20 million to launch a Falcon 9, yet propellant is only ~$200,000 of that, or around 1%.

Via full reusability and simplified operation, SpaceX hopes with Starship to dramatically reduce expenses to the point where it'll cost only a few million to launch 100 t to LEO.

To illustrate, assume a full Starship will cost $10 million to launch (that's five times Musk's most quoted estimate, just to be conservative). Launching 200 kg then costs $20,000. Still high relative to air transport, but it's getting there.