Below My idea about ON reading "BEI".

Plate forms circle and flat at a guess.And it's colored white.
I think resemble sound of "BEI" and "HEI".

KANJI is satisfied above of terms is "幣"and"餅".

幣 is used about 神道（Shinto）originally.
"circle"+"flat"=coin.幣 was meaned money after. ex:紙幣（papermoney)貨幣（coin money)

餅 means rice cake.Do you know it?
餅（KUNreading:MOTI ONreading:HEI)is white,flat and circle.
In this connection,MOTI made from rice.rice is 米（ONreading:BEI KUNreading:KOME)
Icome to think of it,rice also white and flat and circle.

This is my arbitrary idea.
Maybe,sound of HEI or BEI meaned "white""circle""flat"things at early Japanese.

Lastly,I apologize to you for my poor English.