12.2. The average retail sales price mentioned in the second paragraph of section 12.1 shall be used to compute the tax provided for in section 6, until it is replaced by an average retail sales price that the Minister shall determine from time to time by means of a representative statistical sampling of the prices of beer sold in cases of 12 bottles of 341 millilitre capacity, excluding the tax provided for by this Act, in force in the beer retail outlets situated on the Island of Montréal.