So Just tossing this out there for thought and discussion;
Chainmail is 150gp and Breastplate is 200gp both provide +5 AC and both are medium armor, my question is why would I pay 50gp more for something that I cannot get the value from. There is a similar situation for Splint Mail and Banded Mail. I can see an argument for having these various types of armor, at least from a role play perspective.
In ODnD there was a similar situation with all melee weapons doing the same damage and there was a cost variation(daggers cost 3gp, swords 10gp and both did d6).
If there is no significant mechanical difference: I would suggest the prices be the same.
If there is a significant mechanical difference: Please let me know, I missed it!
Thanks for your time!