The opportunity to turn critical hit chance into reliable damage is huge. The idea for why you shouldn't be able to get an 'equivalent' amount of damage in exchange for your crit chance is simply that that would make going for crits instead of damage only feasible if your crit chance was above 70% or thereabouts.
As this is neither an all or nothing or always on skill, the skill is primarily going to be used in three ways:
1. The user will give up their crit in order to raise their damage to the point that instead of leaving an enemy on low health after the exchange, it will kill them.
2. The user will give up a fraction of their crit to raise their damage potential to 1/3 of the enemies remaining health and hope for a crit.
3. The user will reduce their crit to in order to raise their damage to the point where an ally can deal the finishing blow.
Reaching any of these conditions is much easier if you decrease the cost of the skill, and would leave damage personal skills much less valuable on a mercenary than a critical hit personal skill, as the latter would give you more options without a downside.
Also we should consider Wrath and weapon crit as well. Wrath in 5 to 1 allows you to raise your damage by the equivalent of 8 STR. That's almost a third of the stat cap at final promotion. A killing edge gets you 5 extra damage. Consider doubling as well. Mercenaries may not have amazing base speed, but fast mercs certainly exist, and allowing them to gain such a great level of extra damage is too much in my view.
I don't think there needs to be parity between what you give up and what you get with a skill like this because you pick and choose when you do it and to what degree, so having the critical hit cost be a little high for what you get is not a problem to me, and will keep the skill from being overused.
So that's my stance on the 5 to 1/10 to 1 debate. Also if we start it at 10 to 1 and it's deemed too weak, we can always buff it to 5 to 1. This is also true of the reverse but people like buffs more than they like nerfs.