I mean half a million drivers each with a car that say costs $20k over its lifetime is about 10 billion. So like maybe?

$20K is a very low estimate for cost of owning a car. Edmunds estimates the 5-year "True Cost to Own" of a Toyota Camry for home-use is $35K. This is based on a 15K miles a year, Uber drivers can easily put on 50,000+ miles a year.

500,000 drivers * $20,000/car => $10,000,000,000 or $10 bil, not $10 trillion.

Thus far, Uber has raised $24.2 bil in financing so they could have bought cars for every driver easily. No idea what they did with all that money.

Subsidized fares.

Is there a single company in the world that pays for its machinery with cash, upfront? I don't think so.