You can use caddy commercially for free, the same as nginx. You just have to use a copy built from source by someone other than Light Code Labs, because the prebuilt binaries downloaded from the official caddy site are the only ones with the commercial licensing restriction.

I think caddy and nginx are both great pieces of software that have overly expensive commercial pricing.

I had the same impression, but mholt pointed out to me that building from source or using the Github binary releases are valid alternatives to paying for licensing.