I'll make it quick.
your culture, diplomacy skill/political standing with all races, economy, planets owned, and maybe military are put into a equation and when trading with credits for something you may pay more or less for something depending if your score is lower or high than the nation you're trading with.
This is something galciv2 didn't do.
It would be nice though if no matter how many planets you owned the score still was equivalent to a identical larger empire, just saying.
DARCA. 