@ (At Sign) meaning "at a rate of"

QUESTION = The earliest yet discovered reference to the symbol is a religious one;

it features in a Bulgarian translation of a Greek chronicle written by Constantinos Manasses in 1345 .Held today in the Vatican Apostolic Library, it features the symbol in place of the capital letter alpha 'A' in the word Amen. Why it was used in this context is still a mystery.

In terms of the commercial character of the sign, there are several theories pending verification.

ANSWER = @ (At Sign) meaning "at a rate of"