How is ship cost determined?

I've been paying more attention to the cost of certain ships. It would seem obvious that the more slots a ship has, the more expensive it will be. However, there seems to be a lot of inconsistencies with this. The jump from 15 to 20 slots may only be 800,000 Units, but from 20-30 becomes 5 million Units.

Is there a set calculation for how much ships cost? Does affiliation with a race determine how expensive their ships may be?