UNRWA get's 12$ from donors, purchases a bag of flour and delivers it to the Israeli authorities at the border crossings into Gaza.

The people in Gaza then have to collect 600$ from their supporters to withdraw 300$ to purchase that bag from a local merchant.

300$ withdrawal fee are probably split between some "money changer" in Gaza and someone who can bring money into the strip, probably IDF personnel.

300$ for a bag of flour are probably split between the merchant in Gaza and someone looting the aid waiting at the crossings.