I came up with this over in House Rules. Note that it is powerful in that you get permanent effects, but you pay the same experience cost as with slotless wondrous items and you burn a feat that only lets you do certain spells. So instead of slotelss items that can (rarely) be destroyed, you get permanent spells that are tough to lose, but with a very restricted list.
PERMANENT PERMANENCY
Prerequisites: Ability to cast Permanency; ability to cast 7th level arcane spells (including the use of Miracle to cast 7th level arcane spells).
Benefit: Whenever a spell that was made permanent upon the caster's person would otherwise end due to a dispel effect, the magic is instead only suppressed for 1d4 rounds. This benefit only applies to spells the caster has done himself, and only to those spells made permanent after taking this feat. In addition, these permanent spells are treated as items for the purposes of eliminating them so that, for example, they get a Will save vs. Mage’s Disjunction. Finally, the experience cost table is replaced by the table below:
Code:
Spell Name New XP Cost
Arcane sight 5,000
Comprehend languages 250
Darkvision 1,000
Detect magic 200
Read magic 150
See invisibility 2,000
Tongues 4,000
Enlarge person 350
Magic fang 350
Magic fang, greater 2,500
Reduce person 350
Resistance 200
Normal: Permanent spells are subject to being dispelled by any caster one level higher than the caster level when created, and they are automatically disjoined just as any other spell would be. Note that Telepathic Bond must be cast upon two creatures, and so in ineligible for this feat.
The below is a comparison of experience point cost of a slotless wondrous item (use activated or continuous) to Permanency’s cost.
XP = Spell Level x Caster Level x 2,000 x Duration Modifier x 2 (Slotless Modifier)/25
Code:
Spell Name Item Experience Point Formula Item XP Perm XP
Arcane sight (3 x 5 x 2,000 x 2 x 2)/25 4,800 XP 1,500 XP
Comprehend languages (1 x 1 x 2000 x 1.5 x 2)/25 240 XP 500 XP
Darkvision (2 x 3 x 2000 x 1 x 2)/25 960 XP 1,000 XP
Detect magic (1/2 x 1 x 2000 x 2 x 2)/25 160 XP 500 XP
Read magic (1/2 x 1 x 2000 x 1.5 x 2)/25 120 XP 500 XP
See invisibility (2 x 3 x 2000 x 2 x 2)/25 2000 XP 1,000 XP
Tongues (3 x 5 x 2000 x 1.5 x2)/25 3,600 XP 1,500 XP
Enlarge person (1 x 1 x 2000 x 2 x 2)/25 320 XP 500 XP
Magic fang (1 x 1 x 2000 x 2 x 2)/25 320 XP 500 XP
Magic fang, greater (3 x 5 x 2000 x 1 x2)/25 2,400 XP 1,500 XP
Reduce person (1 x 1 x 2000 x 2 x 2)/25 320 XP 500 XP
Resistance (1/2 x 1 x 2000 x 2 x 2)/25 160 XP 500 XP
That's a fascinating table. I assumed that all are double price for slotless and are "always on" (as opposed to "use activated," which would have no duration modifier) and applied the appropriate spell duration modifier.
That table is how I derived the new costs.
As a side note, it shows how screwed up the table is in the book. I'd say regular Permanacy cost should be perhaps 1/4 of a permanent item - since at high levels they will be routinely dispelled - you can count on it. High levels battles often start with Greater Dispel Magic spells going off on both sides.