Minor bug but annoying if you don't realize its there. Easy to reproduce.
Try to buy the first factory on a planet as the Thalans (Super Hive) and it costs 690 BC.
Set the first factory to build, THEN try to buy the factory on the same turn and it only costs about 140 BC.
The bug is that it should cost 140 BC to buy in the first place because of the Thalan Super Hive ability to build factories at 25% cost, but that ability apparently isn't taken into account until you first queue it up to build, then buy it.