But if you look around you can find deals much lower. Envoyer regularly has them for both grower and big houses at the (what I consider) usual markup just above 2x750.
Mags are generally only at a small premium if you buy on release/cellar door/pre arrival etc. Once they get into the retail chain they get a premium for all the aforementioned reasons.
Just requires planning in advance and they are then decent value. As mentioned, Envoyer mags are normally reasonably priced.
In addition, champagne ages better in larger formats. It has to do, apparently, with the larger wine surface, amount of oxygen contact etc that lets larger format bottles age into complexity while retaining freshness.
I was once at a dinner a number of years ago with large format bottles of great Bordeaux going back to the 40s. In addition, there was a Salmanazar of Moet champagne from the 60s. I stuck to the champagne all night as it was superb: complex, aged, fresh.
I believe in those days they put their best lots in the bigger bottles as well, and sold them to VIPs, royalty, presidents etc. But that may be a myth.
I don´t see much difference between Champ.Mags and other Mags.
It´s usually some 10-20% above the 2-bts-price, if at all, sure depending on producer and rarity … but I admit some retailers make additional cash with it.
There is some premium because the larger format bottles just cost more than double, and particularly on champagne, where the glass needs to be extra thick. That accounts for a portion of the multiple, but I’d guess, as others have noted, the rest is demand/prestige/opportunity.
Seven or eight years ago, I had a conversation with Neil deGrasse Tyson about it and he agreed that there would scientifically be an optimal bottle size for Champagne and the hypothesis was that it was magnums.
Gérard Liger-Belair wrote “Uncorked: The Science of Champagne” to answer many of these questions. Published by Princeton and winner of the 2004 Award for Best Professional/Scholarly Book in Physics and Astronomy, Association of American Publishers. Not sure if he goes into bottle size. Talks a lot about bubbles. But of course it was published at that time.