The value of a mitzvah is so great that nothing in this world is sufficient to reward its performance.  In the words of the Vilna Gaon, “There is nothing [in this physical world] with which to pay for a mitzvah.”  Since service of G-d is spiritual, it is a degradation to think that something of this corporeal existence could serve as recompense for it.  ~ The Pirkei Avos Treasury