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.