I'd do a comparison based on the druid I played from 1st -> 15th in a long campaign, but the last char sheet I can find of him is from 12th level just after he lost almost all his gear in an unforunate accident... wouldn't be a particularly good example.
From memory tho I ended with 24 wis (19 + extrapolated +5 wis periapt), wooden wild fullplate +2 (cast ironwood on it once a week or so), animated darkwood shield +3 and an ioun stone of +1 CL... that was pretty much it. Thats somewhat excessively below standard wealth, our party just tended to give items to whoever could use them, then split the cash equally. This didn't particularly bother me as most items wouldn't work in wildshaped form anyway in our campaign... it did make the character a bit dull tho. That and not being able to speak while wildshaped kinda put me off the class a bit.
Anyway in dire bear form (my preferred shape), AC was -1 size, +1 dex, +7 natural, +11 armour, +5 shield + barkskin = 38 total.
Ignoring the periapt and ioun stone, which would be the same in both builds, the 50k spent on the plate+shield would get belt (13k) + bracers of armour +6 (36k). Ring of deflection was ruled to merge while wildshaped in all animal forms but gorilla types, so that wouldn't have been any use. Amulet of natural armour doesn't stack with barkskin, and I had a periapt anyway, so also useless.
Belt build would get me AC = -1 size, +1 dex, +7 natural, +7 wisdom, +1 belt, +6 bracers, +5 barkskin = 36 AC. 2 points lower.
The dex restriction doesn't matter as a bear (13 dex), the movement speed decrease is fairly minimal (40' -> 30'... compared to the party fighter/clerics at 20' or others at 30'), and ACP usually didn't matter. The only significant thing was spending a feat, but even without it you can do the same with a breastplate for a total of 1 AC less than said belt build.
However as you get more money (ie. closer to standard wealth) the belt becomes markedly less effective; you're stuck with improving 1 item (bracers of armour) for extra AC as opposed to 2 (armour, shield)... this is assuming you're going to be maxing your wis bonus anyway, as I did, yet that would be less important with a non-belt build.
+6 -> +8 bracers of armour = 28,000 gold for 2 ac. +5 armour/shield -> +6 = 11,000 gp each for 2 ac. And remember those bracers max out at +9... so eventually you're looking at +5 wild fullplate (+8 equivelant) and +5 animated shield (+7 equiv) for +20 AC vs your +9 bracers and +8 wis bonus for +17 AC. If you dump a ton of money into tomes, you can manage to get +11 wis bonus and equal the armour/shield build. Barely. But it won't happen til 20th, and the majority of campaigns won't get that high.
The monks belt has certain advantages and disadvantages, its certainly not overpowered tho even for the druid, who arguably benefits the most from it. (PS. monks belt won't be too happy when you wildshape into a fire elemental)
Ah-well. Point still stands!
