Ring of Blinking- 27,000gp.
Miss Chance > increased AC, accomplishes the same thing.
I like the argument about the miss chance being better than AC.
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20* Dex to start (+5 bonus to AC)
Mithral Breastplate +5 (29,200gp / +10 bonus)
Shield, Heavy +5 (25,157 gp / +7 bonus)
Ring of Protection +2 (8,000gp / +2 bonus)
Amulet of Natural Armor +2 (8,000gp / +2 bonus)
Ring of Blinking +27,000 (50% miss chance)
=97,357 gp for AC 36 (+21 over the Dex) and 50% miss chance
* If the Dex was only 16, then spend some of the extra money to get a bunch of potions of cat's grace.
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Against an attack bonus of +16 or lower, this is better than having a higher AC because half of the natural 20s still miss.
Against an attack bonus of +17 to +33, the effective AC is 37 + (attack bonus - 17)/2. So, a 37 vs. attack bonus of +17 all the way to 45 against an attack bonus of +33.
Against an attack bonus of 34 or higher, the effective AC is the attack bonus +11.5 (since they hit 9.5 out of 20 times given the auto-miss on a 1). So if you are fighting a Tarrasque, your AC is effectively 68.5 against the bite attack. Also, a natural 20 that otherwise would have been a critical is also concealed away 50% of the time.
Edit: Ack! Forgot the base of 10 for the AC the first time through. Should be right now.