Will the Kohanim keep the money for themselves?
Rashi: They money will be theirs, but they should accept on themselves to fix Bedek ha'Bayis from their money.
What is "Makaro"?
Rashi: It is the one normally found by the Kohen (Radak - and gives to him his Matanos Kehunah).
What is "Bedek ha'Bayis"?
Rashi: It is cracks in the walls.
Rashi (Bereishis 42:9): It is Ra'ua (dilapidation of) ha'Bayis.
Radak: It is where the Bayis is broken or destroyed. It is called so because one must be Bodek (check) to know what is needed.
Ralbag: It is everything needed to strengthen the Bayis, and to check if it is missing anything needed.
Why did he say to put aside money for Bedek ha'Bayis?
Radak (5): It says in Divrei ha'Yamim II, 24:4 that afterwards, he put to his heart to redo Beis Hashem. It needed to be strengthened, for the evil Asalyah and her sons breached it 1 . If not for this, it would not need to be strengthened, for it was built very strong, and it was less than 150 years old. He needed to collect new money, for most of the Shekalim and Hekdeshos in Beis Hashem, Asalyah and her sons made from them idols of Ba'al. Yo'ash did not want to use the remainder, lest Otzar Hashem be empty. 2
Divrei ha'Yamim II, 24:7. Radak (24:7)- these sons were from a previous husband. (Had they been from the king, one of them would have become king! - PF)
Radak: He wanted to do it from new money that would be brought to Beis Hashem. It says in Divrei ha'Yamim II, 24:9 that he announced to bring Mas'as Moshe. These are the Shekalim, based on "Ki Sisa Es Rosh Bnei Yisrael
In Divrei ha'Yamim (II, 24:4-7), it mentions sending Leviyim to cities to gather money. Why is this omitted here?
Malbim: Here it discusses only Bedek ha'Bayis. Yo'ash also wanted to restore the Bayis to its initial beauty. Kesef Over, Nedarim and Nedavos sufficed only for Bedek ha'Bayis. Divrei ha'Yamim tells that he sent the Leviyim to collect to beautify the Bayis.
