The Wealth of Nations by Part 1 Chapter 11 Page 108

Elizabeth, the exportation of wheat was in the same manner prohibited, whenever the price of the quarter should exceed six shillings and eightpence, which did not then contain two pennyworth more silver than the same nominal sum does at present. But it had soon been found that to restrain the exportation of wheat till the price was so very low was, in reality, to prohibit it altogether.

In 1562, therefore, by the 5th of Elizabeth, the exportation of wheat was allowed from certain ports whenever the price of the quarter should not exceed ten shillings, containing nearly the same quantity of silver as the like nominal sum does at present. This price had at this time, therefore, been considered as what is called the moderate and reasonable price of wheat. It agrees nearly with the estimation of the Northumberland book in 1512.