Pirke Avot 2:1

Rabbi taught: ... Be as attentive to a minor mitzvah as to a major one, for we do not know the reward for each mitzvah. Weigh the loss incurred in performing a mitzvah against the gain ...

Rabbi Yehudah HaNasi's words above seem to contradict themselves. One the one hand, he says that we do not know the reward for mitzvot, and on the other hand he suggests balancing any loss we might incur from perfoming a mitzvah against this unknown gain. How are we to perform this balancing act?

One solution comes from Rabbi Yaakov, who is quoted in Pirke Avot 4:22 ... one hour of bliss in the world to come is worth more than all the pleasures of this world. The reward in Olam Haba for each mitzvah is so inconceivably vast that even the reward for a minor one is worthwhile. Do not say that I will only perform the major mitzvot and acquire the awards for them - the reward for even the lightest mitzvah is beyond our imagination. And when we think of the cost of keeping kosher, of not working on Shabbat, or of getting our clothes checked for shatnez, balance that against the infinite reward awaiting us.