I don't know. SR is good, but it's not that good. What with spells like Assay Spell Resistance and True Casting, it's not that hard (though it does cast you actions, daily resources, and possibly long term resources if you're a caster with a limited spell list like Sorcerer) to be able to beat SR, unless it's very very high. And there are plenty of spells (mainly conjurations) and indirect spell-based attacks (telekinesis to throw heavy things, etc) that don't allow SR at all.
It's certainly better to have SR than not to have it (assuming you can lower it to receive friendly spell effects when needed), but unless you have something like SR 30 + ECL it's not an I Win button against enemy casters. Even if you do have SR that high, it's still not an I Win button (maybe against idiot evokers), it just requires them to be more picky about what they come at you with, and probably buys you some time.
Cold iron is already pricey, and at present it doesn't do jack except beat DR. I don't think a major price hike is necessary nor appropriate for allowing cold iron armor that naturally helps its wearer defend against spells. (The price, btw, is that it doubles the base cost of the item, and any magical enhancements cost an additional 2,000 gp.) Maybe a small price hike, but I wouldn't call a price of 30,000 GP "far too low" for a suit of +5 full plate that gives you a lot more than just +13 armor bonus to AC, excluding touch AC. If you go by the standard WBL tables you can't afford that at all until ECL 9. If you're not willing to spend more than half your wealth on it, it's pushed back to ECL 11. If you really have your head on straight and understand that there's a lot more to adventuring than being mobile difficult terrain, and you're not willing to spend more than 1/4 your wealth on a single defensive item, your suit of plate isn't available until somewhere between 13th and 14th levels.
Of course you could just get a +1 full plate and hang special effects on it, and use Magic Vestment to boost its enhancement bonus to +5. That would bring the price of the suit down to 6,000 gp (just out of reach for a 4th level character, and a dangerous amount of a 5th-level character's wealth), and each separate special effect you add would cost an extra 2,000 gp. Trouble is you'd probably have to beat the armor's SR to cast Magic Vestment on it in the first place, and even if you didn't, all you're doing anymore is dragging spells in as the only real defense against spells, which is exactly what cold iron armor is trying to avoid in the first place.