infused armor

where do I go to get my armor infused???

iron mines of moladune mission

Or in Mineral Springs, but it's in a more out-of-the-way location.

thanks

Also for those who already got Infused, there is another place. At the far north east corner of Mineral Springs, inside a cave full of ice imps and Squids.
The Seer is in the cave and will infuse any armor, (mostlikly 15k armor bought from Citadel).
So for those who wanted to infuse a second (or third) set of armor you can still get infused without backtracking.

how much does it cost?

It doesn't cost anything, except you must do something first. I'd rather not spoil it, just continue with the missions. There's only 1 mission where infusion could help you but you won't have it; just continue with the missions, infusion doesn't do much for you at all.

infusion allows u to walk to grotto

I'd like to see you try walking through the mobs there. But yes, it would protect you going from Thunderhead Keep to Marhan's Grotto.