One ought to increase up to 1/3 of the value of a lesser item, in order to purchase a finer item for a Mitzvah: Bava Kama 9a-b
Calculation of 1/3 as inclusive or exclusive of the actual 1/3: Bava Kama 9b
The added 1/3 is rewarded after death, but additions over 1/3 are paid back in this world: Bava Kama 9b
Gd had mercy on the Jews' property [chas HaKadosh Baruch Hu al mammonam shel Yisrael], limiting what they had to spend for a specific mitzvah: Rosh HaShanah 27a; Yoma 39a, 44b; Zevachim 6b
Greater expenditure shows love of the Mitzvah: Succah 41b