Closer to $40b for a new fab for an established company to do it all correctly. It's a much more major investment to open a fab without ever doing it before, then continually use the brain power/institutional knowledge you've built up to stay near the forefront of fab tech, and then basically have weird incentives to build a foundry for only your products rather than the world at large.

You're setting yourself up for making a huge part of your future revenue stream being set aside for ongoing chipfab capex and research engineering. And that's a huge gamble, since getting this all setup is not guaranteed to succeed.

If it were only 20B then Apple would jump at the chance.

As would almost innumerable others.