You would think so, but...
AA is a good example. When they released Windows and Linux versions, the development team was about 30 strong. One guy alone ported the AA client to Linux over numerous releases. So ~1/30 of the dev budget (~3.5%) spent on the Linux client means if Linux users are 3.5% of the userbase, the Linux port costs no more than the Windows version to release.
In fact its better than that, Icculus (the Linux dev) ported the Linux client part-time, because he was also on the Linux Server team (the AA server runs on Linux). Figure his time spent equally between Server and Client versions and you only need ~1.8% of the userbase running Linux to make the port cost effective. Of course development costs are only a part of the costbase for a game (add Marketing etc.) so the real % of Linux users required is far lower.
The argument for a MacOS X version is even more compelling because the market share is higher, releasing all 3 ports offers the best bang for dev buck