An interesting comparison is to reverse engineer the price for constant effect Mind Blank on the +1 Halfweight Illithidwrought plate from Underdark p71. The market price is listed as 146,650. 1650 for MW full plate + 25000 for the (+1 Halfweight Illithidwrought) leaves a balance of 120,000 for constant effect Mind Blank.
Sure, 90,000 gp is too cheap, but consider that 205,000 gp would represent 27% of a 20th level character's suggested wealth. By way of comparison let us examine the Rob of Eyes, which weighs in at a whopping 120,000 gp. Remember that pricing this is an art, not a science.
To extract a likely price analysis, let's take the 120 darkvision & see invisibility to start. See Invisibility is a 2nd level spell with a duration of 10 mins/lvl (3 x 2 x 2000 x 1.5)=18,000. 27,000 gp for 1.5 multiplier on price for multiple effects. Darkvision- 60 ft darkvision is 12,000 gp so doubling for 120 ft and then multiplying by 1.5 would net it at 36,000. The +10 to Search and Spot would be 15,000 +15,000, for a total of 30,000. The drawbacks probably mitigate some of the cost of the Dex to AC retention.
So far 36,000 +30,000 +27000=93,000. I would say 27,000 for the Dex retention is about right considering the drawbacks. If evasion is worth 25,000 by itself in a ring, and is arguably better than improved uncanny dodge, then 27,000 with a couple drawbacks is ok. I mention evasion as a means of pricing class abilities only. It is fielder's choice as to what to designate as the primary power, this is all estimating anyway.
Now, let's try to engineer a price for true seeing. 120 ft darkvision that can see in magical darkness? Ballpark that at 50,000 if you multiply the 36,000 by 1.5 and ad hoc 4,000 off. See Invisibility prices out at 27,000 gp. A crystal ball with true seeing is 80,000 gp, which is fairly comparable. The increase in range with the crystal ball is offset by the 1 hr activation time. The target gets a save, but the user can be in a place of safety. Also, the crystal ball is effectively an "unslotted" magic item, so its price is inflated accordingly. While is has true seeing on it, this one doesn't offer much, although I found it interesting that if you had it be a slotted item it would be 40,000 or 60,000 gp depending on how you did the math.
Personally, I would ballpark this item at 150,000 gp, certainly no higher than 175,000. If slotted Mind Blank is worth only 120,000, then True Seeing, a spell three levels lower should be reasonably close. The cost of 100 True Seeing casts is 25,000 gp, so 120,000 (it is comparable in the immunities it provides) + 25,000 in material component costs +5,000 ad hoc would put it at 150,000. A Tome or Manual +5 is 137,500, and a Staff of Life is 155,750. A would put this item on par with the two of these and not with a weapon +10. That's my 2 cents, feel free to disagree.
