I bought the PGC and GAIA Extension Data Bases for SkySafari 6 Pro.
These are huge libraries : The PGC data base takes up 101 MB on disk, and the GAIA data base takes up 1.4 GB on disk.
That's OK but they have been installed at two different places on my disk : one is related to the App Store and the other in my user library.
It seems to me that this is an unnecessary use of disk space.