The commercial tools are indeed very expensive but the required data files can be as much of a problem. Normally you have to sign a bunch of NDAs (non disclosure agreements) to get your hands on the design rules and standard cell libraries supplied by the foundries and required to make the tools work.

One effort to organize several previously available open source tools into a practical system is OpenLane, which is based on the DARPA OpenRoad project:

https://woset-workshop.github.io/PDFs/2020/a21.pdf

Recently, Google has financed a project where a foundry has made its data files available without any NDAs:

https://github.com/google/skywater-pdk

The combination has made it possible to have completely open source chip designs.