Observation on Dragon Costs

So, I'm working on dragons, and while doing so I noticed that:

Selene costs 300 dragon coins, some mats and flux total.

Azorian (The Azulian dragon) costs 10 dragon coins per soul, so to get to 40 souls (his completion number), he costs 400 dragon coins and some mats total.

All of the other dragons require 4 dragon coins and take 100 souls, and therefore cost 400 dragon coins and some mats total.

Why the discrepancy between dragon costs with the new to the old? Why can't they all cost 300 coins and some mats?