A life for a life. It is the least we can do for Him...

True, there is absolutely nothing we can do to save ourselves. His death on the cross indeed is a gift done purely out of His grace.

The cost to Him was everything but do we not give up something as well?

While it is true that our following Him does nothing to save us it is just as true that if we do not deny ourselves and forsake and bear our cross to follow Him we cannot be His disciple.Jesus said this clearly.  If we do not give up all we cannot say that are we truly following Him.

If any man come to me, and hate not his father, and mother, and wife, and children, and brethren, and sisters, yea, and his own life also, he cannot be my disciple. [27] And whosoever doth not bear his cross, and come after me, cannot be my disciple. [28] For which of you, intending to build a tower, sitteth not down first, and counteth the cost, whether he have sufficient to finish it ? [29] Lest haply, after he hath laid the foundation, and is not able to finish it , all that behold it begin to mock him, [30] Saying, This man began to build, and was not able to finish. [31] Or what king, going to make war against another king, sitteth not down first, and consulteth whether he be able with ten thousand to meet him that cometh against him with twenty thousand? [32] Or else, while the other is yet a great way off, he sendeth an ambassage, and desireth conditions of peace. [33] So likewise, whosoever he be of you that forsaketh not all that he hath, he cannot be my disciple. Luke 14:26-33

Then said Jesus unto his disciples, If any man will come after me, let him deny himself, and take up his cross, and follow me. [25] For whosoever will save his life shall lose it: and whosoever will lose his life for my sake shall find it. [26] For what is a man profited, if he shall gain the whole world, and lose his own soul? or what shall a man give in exchange for his soul? Matthew 16:24-26

He gave His life for us and it makes sense that in return we give our lives to Him.

We don't have to, we get to!

And whosoever of you will be the chiefest, shall be servant of all. [45] For even the Son of man came not to be ministered unto, but to minister, and to give his life a ransom for many. Mark 10:44-45

He gave of Himself to die under the crushing weight of every sin ever committed. Not just my sins but the sins of the whole world.

He paid, in the most literal of sense, everything so that we could be reconciled back to our Heavenly Father.

The least we can do is give our lives to Him.