http://samanna.net/sham.info/aas.shtml
To do full Precog - you would spend 25 aa for 2% more dodgy (if I am reading the chart correctly).
To do CS3 + ID5 - you would spend 21 aa for 21% more mitigation.
Assuming I am reading the chart right

The gains get much smaller and the costs increase as you move higher up the expansions.
I do a lot of healing, so I did all my healing AA before I did Omens/DoD defensive AA, for example (I returned after a long break). Finish up at least radiant cure 3. If you're getting hit a lot, you might want to just go ahead and finish out those defensives. Or better yet, finish up spell casting subtlety, so you don't get hit anymore. I know this might be kind of out of the blue as well, but I also did Pet Affinity fairly early after returning, if you use your pet at all, this is an excellent AA. The pet becomes much more viable when he has high level buffs. I have never regretted spending those 12aa.