I am GMing my first Pathfinder game and a player (a wizard) wants to make robes that offer Protection from Good and Law and Evil and Chaos (as the spells). That seems a bit OP to me.
How would that work and what's the best way to handle this?
There isn't anything limiting all of these spells being on the same person at the same time, so that isn't necessarily a concern. Let's think about what the person actually gets by using an item like this. It would effectively be: (all these bonuses are only against non-neutral neutral (many monsters fit neutral neutral, and definitely golems :D )
This probably isn't OP, as you can always just throw neutral monsters at it and it does no good at all.
Formula For the Item:
So, the formula for a continuous spell effect is spell level * caster level * 2,000 GP * 2 for 1 min/level spell, and each effect beyond the first has a 1.5 multiplier
Honestly, if a player wanted to spend 22k Gold on an item like that, I would be more than willing to oblige them, as there are probably more broken things they could buy for that amount. Just my two cents.
As a point of reference, a cloak of resistance +2 is 4,000 GP, and a ring of protection +2 is 8,000 GP. Are the other effects (plus the fact that they're all on the same item) add up to another 5,000? Probably not. I would say that 22,000 is a minimum price, and just look north of there.
That custom magic robe of alignment protection wouldn't be particularly powerful
The deflection bonus to AC is easily duplicated, as is the resistance bonus on saving throws, and these bonuses won't stack as they're named bonuses. A second saving throw against mental control is only warranted against neutral creatures' effects, other creatures' mental control prevented completely by the robes. Summoned aligned outsiders that want to make bodily contact with the robe wearer will probably be rare unless the campaign's main baddies focus on summoning or something.
Thus if the campaign's focus is on magical possession or summoned creatures—you're playing a campaign based on The Exorcist or Magic: The Gathering, for instance—then such an item is a big deal and likely unbalanced, so the robe should be disallowed or outrageously priced.