I'm still toying with the best way to handle mithril and adamantine with the armor I have in
this post here.
Currently, I have it set to use mithil per the DMG, and adamantine granting an AC bonus and a hefty DR to help compensate for the increased mobility of mithril. That seems to be the problem though. It's so hard to come up with something that make it worth losing the mobility without going completely overboard.
I'm thinking about getting rid of the part of mithril that drops it one category, but leaving in the rest (+2 max Dex, -3 ACF, -10% ASF). I have no problem with armor droping a category, but I'd rather leave that as a class feature and/or feat. This keeps mithril from being that much better than adamantine. Also, this keeps each type (light, medium, and heavy) at pretty much the same strength, so the cost can be a flat +1,000 gp, rather than changing per armor type.
If I do this, I can scale adamantine back to keep it affordable. Now, if mithril adds +2 max dex, then it has the potential to have an AC two higher than mithril. We don't want to make mithril more tanky than adamantine. I think I can add a +2 AC bonus to adamantine armors. I'm also thinking that this AC boost is lost against adamantine weapons (or weapons treated as adamantine for purposes of DR). This keeps the ACs comperable; just one material favors high-dex wearers, and the other favors low-dex wearers. Also, I'm thinking about adding a flat DR 5/adamantine to all categories of armor.
So, how much should those two abilities (AC boost and DR) increase the cost of armor? +5,000? +10,000? I've never been good at pricing things like this. I'm leaning more toward 10,000. What are your thoughts?