I just found when I was upgrading my WWII Cruser's to Aegis Class Cruisers that they are free!

Is it a bug or by design? But whatever it is I don't think they should be free.

http://www.civfanatics.net/uploads6/freeaegis.JPG

That's most likely not a bug, the cruiser probably just costs more than the Aegis cruiser.

They cost the same to build, hence don't cost anything to upgrade. I would class this as a bug, but it's not really.
It's already known about anyways :p