Tries his best 'drcollins voice':
A protection from X cloak is only cheep if the DM allows the guidelines to win over good judgement. Clearly the effects are far in excess of what the item would be priced at with the guidelines. Which is what the charts are: guidelines. They break down very fast when you try to simply apply them directly to spells, particularly when you ignore the duration.
The mentioned cloak should either reproduce the effects of the spell exactly (with regards to having a standard action to activate via command word, having a duration dependent on caster level et cetera), or be priced in accordance with the benefits of the bonuses, not the source.
The DM should price any suggested new item based on his own judgment of it's power using the formulas and existing items as a guide. The formulas are fine for charged spell trigger and spell completion, but wondrous items and rings in particular tend to be troublesome when used to replicate spells without regards to the spell's limitations beyond the spell level (there's a lot more that goes into spell balance, like range, duration et cetera).
Also, I could rant quite a while about persistent spell. It's, IMHO, the most poorly hack-balanced feat in any E3 supplement. Quick case in point (don't mention 3eR haste in response, though. 3e hast is just the best spell I can use to make this point with):
Within the rules, there's nothing to prevent someone from making an alternate form of haste that has personal range. For instance, say I have a 'shadow mage' player who wants to, instead of taking an extra partial action every round, have his shadow detach and act on it's own, but that would otherwise be identical to haste. This is a fairly cool idea, I would just tinker haste a bit, make it personal range and add some minor benefits to compensate for the reduced range and AC boost.
If I allow persistent spell, it's a problem waiting to happen. There's no way researching a version of a spell that's worse [changing any range to a personal / lower fixed range] should make it better, balance wise.
Also, persistent spell has the same failing as magic items. Too many spells are balanced with regards to their durations. Disregarding the original duration completely is a very very very very bad idea for anything short of epic play, IMHO. Persistent would still be a great feat if it only increased the duration by a factor of 10 to 20 (still twice to four times what you get for a 4x extend). The few spells it's actually useful for border on being abusive in almost every case.
