The Glowing Sandstone of Focus is expensive but worth it. 1150 points for an item that puts you outside of most mob's AoE's. Problem is that if you really want to do it right and put this aug into a slot on a really useful item you are looking at 760 points on up. The MMC line of the Bloodied caster gear, 1492 points, is good for this because of the number of slots they provide. Problem is with this aug is, being slot 3 you are giving up a potential slot for a Mana Regen aug, so yet more points spent on more gear, and more dungons.
Good thing about it is when you do get it you can now sit from outside the aggro range and nuke them. Whats even sweeter is when the mob is snared and getting pelted by arrows and getting wacked by a tank when its walking twords you.
I would also highly recommend this item to not only nukers but also to healers for the same reasons, avoiding the heal aggro.
Korosha