I don't know for sure... but I've heard it said that sage healers can go as low as 900 expertise in exchange for End / Willpower.
I do know that too much expertise won't help your healing numbers as much as more willpower / power will.
I also know that you should be building your healer around a guard; in other words, make a tank friend and play with him. That'll get the best results.
For the reason above, you can lower expertise if you're in a premade or with a tank.
I get decent results with 920 expertise and 20400 HP (about 1800 WP and 1800 End).