I think looking at the cost of other magical items is not the way to go.
The costs in the DMG are just plain all over the place. Plus, the cost for class abilities or even feats should be extremely high.
I think the key phrase here is:
"as if she had the evasion ability"
What does this really mean?
To me, it means all of the advantages and disadvantages of the ability, not just the advantages.
For example, if we were talking about a Ring of Gaseous Form, we really would not be talking about creatures who could be both gaseous and could attack into the normal plane of existence.
No, we would be talking about what is possible for PCs with the spell.
So, to me, we are talking about what is possible for PCs with the ability of Evasion.
Going beyond this just appears to be incorrect.
You have to remember that a sentence such as "Rogues and monks cannot use evasion in medium or heavy armor.” is not in any way meant to handle every single case. In fact, Shadowdancers (who are also limited to light armor with Evasion) are not mentioned, nor is the Ring of Evasion. Does that mean that you can assume that they do not have the same Evasion advantages and disadvantages because this one sentence in the DMG did not explicitly call them out?
I don’t think so.
To me, that is just semantics. I think that creatures who explicitly call out that they do not have the limitation do not. Everyone else in every other circumstance does. To me, it’s not a property of the ability, but rather how non-light armor affects that ability. It’s like damage reduction. It’s not that the creature is not resistant to damage, it’s that a powerful enough magical weapon will ignore that resistance. It’s not that the ring does not give the character Evasion, it’s that heavy enough armor prevents Evasion from working, just like it limits the character’s speed.
In fact, the sentence “Some creatures with the innate evasion ability do not face this limitation” implies an exception to the normal rule and that most creatures with the innate evasion ability (and every other possible case) do indeed face this limitation.
JMO, but nothing else makes sense. YMMV.
