For Rek'Sai legend do you have to pay the cost to play the unit that was revealed from the legend ability?

Yes, you must pay the full cost of the unit when using the Rek'Sai (Void Burrower) legend ability.

According to RiftJudge FAQ #9215 and FAQ #284, you are required to pay the card's energy and power costs as normal when you play it using this ability. Only effects that explicitly state "ignoring its cost" allow you to play cards for free.