A bit of clarification before launching in full bore. AC is supposed to function as a miss chance of sorts. That is to say, a certain percentage of hits fail to connect, or bounce off the armor, or whatever other fluff description you want that results in it either being a hit or a miss. For a few levels it does that. But offense scales much faster than defense. Between that, the natural 1 = auto miss rule, and Power Attack being an absolute necessity it instead works out like the defense systems in a wide variety of video games.
Everyone has a small innate evade chance, in this case represented by enemies rolling 1s. It is possible to increase your evade via a few means. Aside from this, you are always hit. Defense serves as a buffer of sorts. High defense means you take lower damage and vice versa. Thing about this system is it only works when you can get a high enough defense to be relevant or enemies don't have enough offensive power to tear you apart in very short order or it is not so expensive and inefficient to boost your defense as to not be worth trying.
D&D fails on all counts. Defense as defined by AC is incredibly expensive to get and you still get auto hit and torn apart in two rounds even if you've went well out of your way to focus on it thereby requiring you to either one round the enemy or have a Cleric behind you casting Heal every round and hope they don't just kill him instead. Even then, Heal doesn't keep up forever. This of course is due to the fact the enemies have such high offense your defense specialization doesn't matter. End result is your defense is not relevant.
Now, most video games are at least somewhat balanced in this regard, as a result keeping armor, helms, whatever up to date isn't a problem. A few however are not. In such games, the only valid options for defense are the ones that result in not getting hit. That means overwhelming alpha strikes to win on round 1, miss chances to get some evade in, and so forth. This is where D&D is.
Now, the solution. First, armors and shields can go up to +9 in basic armor and +14 total. You are still capped at +9 for special properties. You can also get rings of up to +6, and amulets of up to +6.
Second, adjust Magic Vestment to be +1/2 levels, max +9 at CL 18th. Adjust Barkskin and Shield of Faith to be +1/3 levels, max +6 at CL 18th.
Third, reduce the cost of armors and shields by 40% and the rings and amulets by 25%, otherwise following the same formula. It may be necessary to further reduce the cost, the first draft is intentionally conservative.
That means armor and shield costs go as follows:
+1: 600.
+2: 2,400.
+3: 5,400.
+4: 9.6k.
+5: 15k.
+6: 21.6k.
+7: 29.4k.
+8: 38.4k.
+9: 48.6k.
+10: 60k.
+11: 72.6k.
+12: 86.4k.
+13: 101.4k.
+14: 117.6k.
This means that for roughly the cost of a +10 armor or shield you can get +13. It also means a maxed properties armor is +5 lots of special stuff, and not +1 lots of special stuff. End result is your ability to improve AC via these mediums scales better and caps higher, thereby allowing it to be more than a yes/no stat.
Now, the other miscellaneous AC boosters:
+1: 1,500.
+2: 6,000.
+3: 13,500.
+4: 24k.
+5: 37.5k.
+6: 54k.
Now, +6 items are available for roughly the same cost as +5s were before, again helping defense to scale better and cap higher. The effects are intentionally lower than those granted by armor and shields for a variety of reasons, not the least of which is that the aforementioned two items are much bigger sources and therefore need bigger improvements to maintain the same rough ratio. Also, there needs to be a point to said items.
As this only effects magic items and spells that simulate them, it doesn't change the low levels where AC works as intended instead of as a yes/no stat. It also has a lesser effect in the mid levels where there is a lessened need for adjustment due to the fact it is not as bad just yet. By picking this curve, the problem becomes self correcting to a great extent. It just goes where it is needed on its own.
Another effect of this is that it only costs slightly more to keep an armor and a shield at level x than it did to keep one of the above at level x before. This helps AC to have a point, and helps shields to have a point. Yay for incidental benefits.
Weapon costs obviously stay the same, as it is clearly counterproductive to fix 'offense scales faster than defense' then turn around and speed up offense as well.