If you pin them to 5 rounds, sure.
Mags are more expensive here because everything is more expensive here. The principles of international trade and equalization of prices don't apply to Canada and US trade. In reality, the difference between US and Canadian prices ought to be the cost of shipping.
Alas, due to artificial economic barriers (import regulations: quotas, duties, fees, etc.) the prices do not equalize as the law of supply and demand shows. In a free-trade economy, the prices for goods tend to equalize because the consumers transfer their demand to where the best prices are. In other words, you import from a country where the price is lower. Local demand subsequently falls because nobody buys locally and local prices consequently fall. Then, foreign prices rise because the demand for their local products is higher due to exports (foreigners buying their goods). Finally, the prices should shift up and down respectively until the difference between both prices is the cost of shipping.
Because of artificial economic barriers at the border, this does not happen.