What is the 'reward' that Jesus is referring to in Matthew 6:6? But when you pray, go into your room, close the door and pray to your Father, who is unseen. Then your Father, who sees what is done in secret, will reward you. - Matthew 6:6 NIV


I would like to think that 'reward' refers to answered prayers. But 'true' believers of Christ should know that God answers prayer (James 5:16), so it seems unlikely it is referring to just that. What is an overview of different interpretations of what 'reward' in this verse is referring to.
One commentator on scripture presents the reward as Grace.  If you view prayer as a sincere communication with God, in humility, the commentary comes to a rational conclusion.  Responding to God's call will include communication with God, in prayer and in other ways.  


  {Catechism of the Catholic Church 1996} Grace is favor, the free and undeserved help that God gives us to respond to his call to become children of God, adoptive sons, partakers of the divine nature and of eternal life.


Matthew Henry's Concise Commentary


  6:5-8 You may as soon find a living man that does not breathe, as a living Christian that does not pray. If prayerless, then graceless. The Scribes and Pharisees were guilty of two great faults in prayer, vain-glory and vain repetitions. Verily they have their reward; if in so great a matter as is between us and God, when we are at prayer, we can look to so poor a thing as the praise of men, it is just that it should be all our reward. Yet there is not a secret, sudden breathing after God, but he observes it. It is called a reward, but it is of grace, not of debt; what merit can there be in begging? If he does not give his people what they ask, it is because he knows they do not need it, and that it is not for their good. So far is God from being wrought upon by the length or words of our prayers, that the most powerful intercessions are those which are made with groanings that cannot be uttered. Let us well study what is shown of the frame of mind in which our prayers should be offered, and learn daily from Christ how to pray.