mellored
Legend
If it is a fireball against a horde of low level creatures it may be a lot more. A hold person against few higher level creatures is also a lot more So high initiative is especially good for casters and against them. Against othe wepon users who damage more steadily it is not too important.
(although a sharpshooter may kill one or two creatures before they even act and thus reduci g incoming damage a lot. Maybe you should value high initiative as a damage prevention tool rather than an increase damage tool.
Any class that has a nova round (fireball, action surge, hold person, stunning strike), is going to have a bigger result from going first.
Which, now that i think about it, is every class but the barbarian, and most rogues.
Also, enemy power usually diminishes. If you face 4 enemies, kill 1 per turn, then you take 4 attacks +3 attacks +2 attacks +1 attack =10 damage if you loose, but 3+2+1=6 if you win. Hard to say if that's a reasonable assumption or not, but probably closer then my initial estimate.
So.. 40% less damage * 4% chance to win * 1.5 pulled out of my

2.4% less damage per initiative.
Making alert (*5 = 12%) pretty balanced with +2 Con (10% if you have 10HP per level).
Unless your a barbarian or non-assassin rogues. Then +2 Con.