"Reaction: after an elite unit is destroyed, put it into play from your discard pile at your hq"

Does that mean i must pay the unit, or it comes back for free?

Pd: sorry if my english isnt very good.

Put Into Play = It comes in for free

If there's an effect that let's you play a unit, then you'd pay the cost.

(Side note: if you ever check out any of FFG's other LCGs, they tend to have some common rules terminology and this is one of them).

Minor note: Conquest uses Deploy instead of Play (which is the equivalent term in Star Wars), but they mean the same thing = pay.