Let's see:
Armor for Unusual Creatures: The cost of armor
for nonhumanoid creatures, as well as for creatures who are neither Small nor Medium, varies (see Chapter 6). The cost of the masterwork quality and any magical enhancement remains the same.
Ok, off to chapter 6
From table 6-8, large non-humaniod is cost x4, weight x2.
The base armor "Breastplate" is a medium armor costing 200 gp and weights 30 lbs. From the rules for special materials, mithral medium armor is +4000 gp, making a standard mithral breastplate cost 4200 and weight 15 lbs.
If we now multiply by 4 and 2, it costs you 16800 gp and weighs 30 lbs.
I think you got your numbers by multiplying the base first (800gp and 60lbs) and then adding 4000 or 8000.
It's possible (but not likely) that large barding is not armor(humaniod), but "other items" in wich case it's (60x500 = 30,000) Yikes !
You're just going to have to ask your GM. I'll look some more to see if I can find anything more about it in RAW.
EDIT1: I've searched UE for barding, but didn't find any that's constructed from special materials.
EDIT2: I've found this thread but no definitive answer was arrived at.
EDIT3: Nothing in Knights of the Inner Sea either.
EDIT4: This thread argues that since nothing is mentioned in the special materials section about size, that the special materials cost remains the same (although that leads to oddities like a tiny suit of adamantine plate costing the same as a regular one).
EDIT5: This one too.
FINAL EDIT: maybe my search-fu is weak, but I cannot find a definitive answer to this. I was hoping to find a set of barding made out of special materials in some adventure path somewhere, but so far I have failed.