Where is the least expensive place to purchase whisky (peaty Scotch) when arriving in France at CDG? (no worries, French wine will be purchased in locale)
Generally a big box retailer such as Carrefour, Auchan, Géant, Leclerc, or Costco. Wine is also available at these locations and sometimes cheaper than it can be purchased elsewhere, including form the wineries themselves.
Whiskey is Irish. Whisky is from Scotland. Either can be purchased in French supermarkets and this will most likely be the least expensive place to buy it.
I second the big supermarkets. There is a big one at Place d'Italie where we stay and we found the cremant we like that is about $20 in the US for 7.50 Euro there. They had lots of brands of hard liquor as well including bourbons. What you want for maximum selection is the giant supermarkets; the neighborhood places will have options but are more limited in choice. I think the one at the Italie Due mall is a Carrefour. The really big Monoprix like Rue Levis or the one by Montparnasse station are probably a good bet as well.