This quote is from Sean K Reynolds page:
Pricing a magic item requires you to be familiar with the pricing guidelines on page 242 of the DMG (or page 74 of Tome & Blood, which takes into account the DMG errata and clarifies some points). The mistake that a lot of people make is using the guidelines as a hard-and-fast rule for prices. Any item that is created should be compared to similar items in the DMG, and common sense should be a factor as well.
This is pretty much said in the DMG as well. But the first step is always to figure out how it compares balance wise to items already in the book.
Price: +4 Armor AC: (4*4*1000=) 16,000gp
+2 Natural Armor AC: (2*2*2000=) 8,000 gp
+2 Defection AC: (2*2*2000=) 8,000 gp
+2 Luck AC: (2*2*2500=) 10,000 gp
+2 Sacred AC (2*2*2500=) 10,000 gp
price is simply sum of property costs = 52,000 gp
Ok, these prices look good. For 5 different items. Now, the fact that combining them into one slot means that they are EASILY more useful than buying the items each individually. Mainly because all you need to do is get an item that gives you an insight bonus to your ac, and you have lots of slots left to put it in. Or, buy a suit of full plate (if this wasn't a monk) and you can have a lot more AC.
You are right that bracers of armor +12 ARE in fact more powerful than this, but not by much. Your touch ac will be higher this way, and with the savings you are getting, you can probably afford some other item that will up your ac (like bracers of armor +8 for instance). Sean K Reynolds mentions this is his discussion, and it says in the DMG, but slots are commodities. Balance is partially maintained simply by running out of places to put new magic items on your body. What you are basically getting from this magic item is bracers of armor +4 and a bunch of other items that don't take up a slot.
Because of this, the best thing to do would be to apply the unslotted cost to everything else other than the primary ability. I also agree that applying similar is not a good idea, as there is no good reason for these bonuses (most of which are given by items already written in the book) to be CHEAPER when all on one item. So, without similar cost and applying the slotless cost that makes it around 70k. However, I agree that since the normal cost of Bracers of Armor (without applying the epic modifier) is around 144k, and this item is not THAT much worse, I'd have to eyeball it a bit more. Maybe 10 or 20 percent worse. For the most part, the practical difference is not much, the monk will be so much better than if he had bought items that are in the book. If we gave it the max discount I can see giving it, that reduces the cost from a 144k bracers to 120k. Seems pretty fair, considering. You have +12 armor with the advantage that you have lots of slots open and the disadvantage that you cannot stack very many items, however, you only need 1 or 2 more good items to get way too powerful ac, so that isn't much of a limiting factor.