Also note that the cost can vary wildly for different games.
Some projects are well written, with all the platform-specific pieces located in convenient locations, and the file formats all have converters to the platform specific rules. There are minimal places in the code where hardware quirks are addressed outside of those platform-specific markers.
Some projects are a nightmare to port. The changes are scattered everywhere in code, with things like data flows through the hardware needing to be addressed in thousands of places. Entirely new toolchains need to be created. The code may rely on subtle behavior changes and side effects in other areas of code, which may take extensive effort and time to decode and address.
Changing a project under development to target a different hardware generation may range anywhere from a minor change (particularly if the project is early in development) to a major undertaking (particularly late in development).
There is no specific number. Hodgman's example of six man months per platform for that specific game may not apply to another project. Another project may require 2 or 10 or 20 or 50 man months per platform. For instance, if you were using Unity or Unreal as your engine for the platform and the code was designed to stick entirely within the engine's functionality, it may be less than a single man month to do the work since the engine directly supports everything. The number can range from a few hundred dollars (where it happens to just work perfectly) to several million dollars (where extensive work is required).
As the saying goes, the devil is in the details.
Also quoting this because it is an important question:
What's the real reason that you're asking this? You've got a prev-gen game that you want ported to current-gen?