§ Rabbi Zeira says that Rav Huna says: For the purchase of an object with which to fulfill a mitzva, one should spend up to one-third.

In the West, Eretz Yisrael, they say in the name of Rabbi Zeira: Of the money one spends on purchasing a more beautiful item to perform a mitzva, up to one-third more than the cost of a standard item comes from his own finances, but from this point forward, any additional sum spent on purchasing a more beautiful item comes from the largesse of the Holy One, Blessed be He, i.e., God will reimburse him for spending that additional sum.

One must pursue beautification of the mitzvah when buying an etrog: If one bought an etrog that can just barely fulfill the obligation, i.e. if it was exactly an egg-size, and afterward found a bigger one, it is a mitzvah to pay up to a third more than the price of the first to swap it out for the nicer one. There is one who says that if there are two etrogs available for purchase, one nicer than the other, the nicer one should be purchased if it is not more than a third more expensive than the other one. RAMA: Somebody who doesn't have an etrog or some other [objected associated with a] mitzvah whose time will elapse need not dispense much wealth on it, as they said, "One who dispenses [money to charity] should not dispense more than one-fifth [of their wealth]" [Ketubot 50a], even with respect to a mitzvah with a time that will elapse (Ros"h, Rabbeinu Yerucham 13:2). This specifically applies to a positive mitzvah. However, regarding a negative mitzvah, one should spend all of their wealth rather than sin (Rashb"a and Raavad). See the end of 658 in the Rama.

From the Holy One, Blessed be He. Rashi says that Hashem will pay him back the additional money he spent in his lifetime. Tosafot is uncomfortable with this explanation because ultimately it is better to be rewarded in the next world where reward is eternal than in this passing existence. The Mishna at the beginning of Masechet Pai’oh says that there are certain mitzvos for which the principal reward is in the world to come and only the dividends are paid in this world. Tosafot holds that the same applies here.
He eats the dividends in this world and the principal remains for him in the world to come.

There was an incident involving Rabban Gamliel, and Rabbi Yehoshua, and Rabbi Elazar ben Azarya, and Rabbi Akiva, who were all traveling on a ship during the festival of Sukkotand only Rabban Gamliel had a lulav, which he had bought for one thousand zuz. Rabban Gamliel took it and fulfilled his obligation with it and then gave it to Rabbi Yehoshua as a gift. Rabbi Yehoshua took it and fulfilled his obligation with it and gave it to Rabbi Elazar ben Azarya as a gift. Rabbi Elazar ben Azarya took it and fulfilled his obligation with it and gave it to Rabbi Akiva as a gift. Rabbi Akiva took it and fulfilled his obligation with it and returned it to Rabban Gamliel.