9th-level Druid build, 200 damage/round

[Iku Rex]

"Aha! You're a doppleganger posing as Bob! You're Monstrous Humanoids, so you have to pay double despite being exactly the same body shape!"

Surely you're not insinuating that this is, through some bizarre twist of logic, silly?

Anyway, I just remembered something. Wild shape doesn't change your creature type anymore! (Doh!)

Good news for the shapeshifters in Infiniti2000's campaign: No extra cost for new armor beyond the increase for Large size. Want an armor for, say, a giant octopus? No worries - same price. The creature type is all that matters.

Bad news for the shapeshifters in Infiniti2000's campaign: Even though their bodies are the same shape as before, their current armor no longer fits. For some perfectly sensible reason.

 

[Infiniti2000]

You offered two options and demanded that I choose one. In fact those were not the only available options. Sounds like textbook false dilemma to me.
How about:
3. The rule says "humanoid" so therefore it's referring to general body shape or creature type.

Eh? How is that not one of the two I provided? Should I have said "pick one or both"? Would that match your desire then? Quite frankly, I thought it was self-evident that you could pick both if you really wanted.

Two creatures with 100% (outwardly) identical bodies.

Well, that's just not possible, but maybe we're talking about different things. You can't have a giant be 100% identical to a humanoid. Even a monstrous humanoid is not the same. If they were, they'd be humanoids. Sheesh. The wild shape errata aside (which is stupid and I don't use), a monstrous humanoid, an ape, a giant, or anything else like that has to pay the x4 penalty for armor. If there's a magical ability that allows someone to exactly mimic a humanoid, then okay but that's not what we're talking about.

We're talking about a stock cave troll. I can't go for that, no can do.

Tell me what humanoid a cave troll is "100% (outwardly) identical" to.

 

[Rystil Arden]

Infiniti--I actually agree with your argument for things that are out-of-proportion like a cave troll would be, but I don't see why a Nymph, an Elan, or a Doppleganger would have to pay double for their armour.

 

[Infiniti2000]

Infiniti--I actually agree with your argument for things that are out-of-proportion like a cave troll would be, but I don't see why a Nymph, an Elan, or a Doppleganger would have to pay double for their armour.

If I were DM, I could be convinced on a case-by-case basis. If that means I concede, then I happily do so.

I'll buy the next round for my penance.