For reference, here's the option of using Spell Resistance Armor from the core game. As you mention, these items do seem a bit overpriced as I found out below.
Suppose we go with it, you can get the following:
BASE ENHANCEMENT BONUS SPECIAL ABILITY TOTAL PRICE
------------------------- ---------------------------- ------ ---------
+1 SR 13, +2 bonus equivalent +3 9.000 gp
+1 SR 15, +3 bonus equivalent +4 16.000 gp
+1 SR 17, +4 bonus equivalent +5 25.000 gp
+1 SR 19, +5 bonus equivalent +6 36.000 gp
To find out how good this is, let's relate this to the wealth by level table:
LEVEL WEALTH LEVEL WEALTH
------- ---------- ------- ----------
1st :) gp 11th 66,000 gp
2nd 900 gp 12th 88,000 gp
3rd 2,700 gp 13th 110,000 gp
4th 5,400 gp 14th 150,000 gp
5th 9,000 gp 15th 200,000 gp
6th 13,000 gp 16th 260,000 gp
7th 19,000 gp 17th 340,000 gp
8th 27,000 gp 18th 440,000 gp
9th 36,000 gp 19th 580,000 gp
10th 49,000 gp 20th 760,000 gp
If you're building a character around this SR concept, let's say you spend about 1/3 of your money on the SR-armor-item. That would mean you'd get the items at certain levels. Also suppose you'll be combatting against an NPC Wizard of your own level:
RESISTANCE ATTAINED AT LEVEL SPELL FAILURE FOR NPC WIZARD AT THAT LEVEL
------------- ------------------- ------------------------------------------
SR 13 8 20 %
SR 15 10 20 %
SR 17 12 20 %
SR 19 13 25 %
I guess that's decent but not great: having about 1 in 5 chance of deflecting a spell with just resistance. If we continue this however, the armor becomes less useful against opponents of equal level:
SR 19 14 20 %
SR 19 15 15 %
SR 19 16 10 %
SR 19 17 5 %
SR 19 18 0 %
SR 19 19 0 %
SR 19 20 0 %
The above shows stats against opponents of equal level. The other situations:
- Many weak opponents will have a harder time beating your SR. If you encounter six lvl 5 wizards at level 10 they'll all have 45% failure.
- One stronger opponent will outlevel your SR quickly. Any one with a caster level of 4 or more above your own level will just about laugh off your SR.
All in all not a terrible option to go with SR items, but not entirely useless either.
Note that using a shield instead of armor only matters a little bit, gaining the mentioned SR-levels only about 1 or 2 levels earlier.
All this does assume you can somehow easily "upgrade" to the next level of SR. In our games the DM usually allows upgrading from one level of an item modifier the next by paying the gold difference to a crafter, which may help in progressing the SR along with your character.