Also English language books are going to be predominantly sold in English speaking countries. And it makes sense cost wise to target both markets together rather than printing individual runs priced for individual countries where possible.

And Canada and the US both use dollars; there is potential ambiguity in price for manufacturers and retailers, so better to clearly specify both. There is not ambiguity with the peso - if a customer sees a book is $10 they will know there is a different price in pesos.