Usamah bin Zaid (May Allah be pleased with them) reported: The Messenger of Allah (PBUH) said, "He who is favoured by another and says to his benefactor: `Jazak-Allah khairan (may Allah reward you well) indeed praised (the benefactor) satisfactorily."[At-Tirmidhi].
If one is unable to return someone's kindness with kindness, he should say, "Jazak-Allahu khairan (may Allah reward you well)." This means: "I am unable to reciprocate your kindness. May Allah grant you the best reward for it." Obviously there is no match for the reward that one gets from Allah. For this reason the Prophet (PBUH) said that it was the best of the praise for a benefactor.