I've worked power-user type applications that are mainly keyboard driven, and those are pretty easy to make cross platform with a single custom codebase that ha

I've worked power-user type applications that are mainly keyboard driven, and those are pretty easy to make cross platform with a single custom codebase that has a platform-specific graphics backend (OpenGL covers many platforms) and minimal platform code. Let's say < 300 lines of boilerplate per platform + 600 lines for OpenGL + 2-5K lines for (shared) UI controls.