Just to provide a rough estimate of the taxes we're talking about, here's what you'd have to pay at YVR (which obviously neither QR or EK fly to, but these numbers and policies should be fairly representative for YUL as well, in addition to plenty of other European jurisdictions).

A bottle of 2013 Dom Perignon costs $240 wholesale, of which $115 is from various liquor taxes (an airline wouldn't pay any net sales tax).

A bottle of Moet Brut costs $59, with $29 of that being liquor tax.

You're paying an additional $15 or so in taxes per 125mL serving (in addition to the increased cost of the more expensive product)

I think what's more important than the taxes, is that you also need to actually buy the product from the local government monopoly. You can't just open a bottle that was bought elsewhere and wire the tax payment somewhere. A government liquor monopoly generally does not allow discounting product or having different prices for different customers. LVMH gets $125 for every bottle of DP sold in BC, but given how the luxury goods market works, I'd be surprised if product to be consumed in flight in first class isn't supplied for free as part of a larger promotional agreement between EK and LVMH.

So your pre departure Moet costs the airline $10 per passenger (whereas a pre departure DP would cost $40 per passenger), and that's almost certainly the drink with the highest cost to the airline of the whole flight.