Ok, the topic is the question.
I am looking for a rule of thumb for mage PvP gear.
In the pre pvp lvl grinde I established, with some short calculation, a simple rule:

-2 spell crit. = 1 spell damage, the rest at will, intuition.

Now, this rule is obviously way to simple for end game fine tuning.
I am open for any advice, here are my first thoughts. Intressting stats are: resilience, spell crit, spell hit (at least till you hit the, what, 3%?), spell damage, stamina, intelligence and mana reg.. You may also look into armor and resistance.
The goal would be to put all those stats into a rule of thumb, so one can evalute items, gems, enchants probably, out of a PvP point of view.

Crit. of course plays a diffent role in PvP vs. PvE. The peak damage is more important but on the other hand, due to resilience it doesn`t bring as much extra damage. So Crit. bs. Spelldamage needs to be looked at, keeping in mind that Crit. upgrades with higher spell damage base.

The main question would be where to but the deffensive stats vs. offensive stats.

Armor and resistance could be calced in damage reduction, and may be halfed, because one only goes against physical, the other vs. spell damage.
I find comparing resilience and stamina also complicated. Since most items that are relevant include those a small sum up:
I assume resilience is linear in its changing the crit. chance and crit. damage reduction. Further I assume 1% crit. raises incoming dps by 1% (so extra crit. damage is assumed to be 100%, this is a very raw assumption, since the crit. damage largly differs in classes and talents) without any resilience, this assumption is well disputable, and one could look closer into it.
I can not come up with a perfect formula, because the two resilience stats change the effectivness of each other. For example, 1980 resilience (oO if its linear, I doubt it though) would mean a 100% reduction in crit. damage, rendering reduced crit. chance completly useless.
Anyways, I will aproximate damage reduction with crit. chance reduction + halfed crit. damage reduction:

- 1 resilience = 0,05075% overall damage reduction (WRONG!!!)

Resilience also reduces volatility of incoming damage, thru buffering peak damage, which is also very important due to limited life, If you hit 0 life just ONCE, it is over..., there is no credit there ;-). So this already introduces life=stamina into the equation.
While resilience reduces incoming overall damage and buffers peak damage, stamina will define the absolut amount of damage you can absorb without any heal, no matter of the dps . we will need some intuitiv aproximations here.
We could define a max dps coming in, depending on what, and against who you play. For example 2000 dps over a couple of seconds in 5v5 are very possible, in a 2v2 2 ap mages could also do that, going up to what ever, 8k dps...
So when you get a steady dps incoming, resilience will hlp. your healer, reducing the damage, while stamina is almost useless, when the max dps drops in, 1% more life and 1% damage reduction will both be on relativly equal terms, basicly making you survive those 1% seconds longer.
So we keep in mind that resilience gets a bonus for peak damage reduction and the duration game, in the crunch time they would be worth the same.
Using 9k life as base:

-1 stamina = 0,1111% more life

So in the crunch time we could sayn 1 Sta = 2 Resilience.
Out of the stomach, i would say, considering the added utility of resilience and the fact that ice block mages are usually not the "to drop target" number one, you may go up to 2 sta = 1 resilience.

Back to deff vs. off stats..., I am lost there!

I do not play competivly in any team, being average skilled, badly geard, stuck on a pve server due to rl friends, and giving up on the hard core gaming, back a year ago. I even lack expierence with the mage and my rl friends just play pve and i drag them into the arena by force, more or less...^^
So input would be great, and apreciated, if you have good theorycraft links, good critics, even the better.

Some other usual stats:

-IG, translates into 1 IG = 0,01231 % crit PLUS added mana, which is a complicated issue itself.

-any mana reg., see above...

-armor: can not be bothered right now

-resistances: hardly exist other than thru talents, intressting though.

Edit: Had some mistakes in it, and worked it over.

Edit2: More and more mistakes are showing up, forget this post for now!

For PVP, it is so situational that I see little point in devising a system of equivalences such as this.

Your team makeup and team role will change the value of these stats to you, as well as each individual match.

While this is definitly true concerning the balance between off and deff stats, the rules of thumb for the off and deff stats itself should definitly hold for any setup.

Even then... depending on your group makeup, you might get focused with different types of damage, by different numbers of people, or not at all even. If a crit heavy class is attacking you, resil will be more beneficial, otherwise stam, etc.

The same goes for dmg... if you're a frost mage, teamed with another frost mage, crit is less important since your targets will be frozen more often.

Not really enough, to change a rule of thumb. Resilience will reduce the incoming damage any way, as long as the oponents have more crit rating than you have reduction. So stamina and resilience will do the same for you.
And even with the extrem example of two frost mages, crit rating will bring the same amount of extra dps, as long as you do not hit the cap.
After all I am talking about a rule of thumb, not an exact calculation for any situation.

another quirky thing (that annoyed me to no end whenever I was fire specced) is ignites and how they work with absorb shields. If I crit a scorch for 1300 dmg and 1100 of it gets absorbed, ignite only ticks on the 200 dmg that was not absorbed instead of the full 1300. Not a huge deal, but I always found it annoying, especially against priests.

People spec AP Frost? I've seen MAYBE 4 Mages that were AP/Frost since arenas were released in pretty much all the brackets.

AP/Frost is the second most popular build amongst high ranked mages according to geekboys (I'm not counting 10/48/3 since thats obviously just people switching for pve).

Eh? 33/0/28 and it's variations has been the most popular build since the arena season started. Probably see a big shift to deep frost with Hypothermia and Mass Dispel changes though.

Guess not on Nightfall then, most of the AP/Frost Mages i've seen are inside battlegrounds, not arenas.