Hacker News Reader

C# or ASP.NET Core never costed anything. The cost is in certain versions of Visual Studio, but that is not required. Besides Visual Studio, Visual Studio Code is popular too.

I can recommend Jetbrains Rider [1] for anything C#. I prefer it over Visual Studio even.

[1]: https://www.jetbrains.com/rider/

Is it also free to deploy? Can you deploy it on Linux?

Yes. It is also fairly straightforward to dockerize dotnet core apps[0].

[0] https://docs.docker.com/engine/examples/dotnetcore/

It's free in all the ways

The C# compiler roslyn, the dotnet runtime (Jit compiler, ...), and ASP NET Core are all MIT or Apache licensed and run officially supported on mac os, linux and windows.