Base 0.5
HF1 - 0.65 (total cost 2 AP)
HF2 - 0.7 (total cost 6 AP)
HF3 - 0.75 (total cost 12 AP)
As you can see, there's a pretty serious diminishing returns effect on Healer's Friend. Every WF that isn't solely in an all-WF static group or all-WF guild should take HF1, but HF2 and HF3 are expensive luxuries.