DF is by and large a rip off, always has been, always will be. The one exception is tobacco which Mrs ATN still uses. Given UK excise duties they are a bargain against UK retail prices.
The DFS at Eurotunnel, Cocquelles is chock full of crappy Australian, American etc UK branded industrially vinified "alcoholic Ribena" retailing at £1 or so below Tesco prices, so more expensive than said supermarket when they have their 25% off offer running. Little French wine other than big brands like Calvet.
We generally get through the DFS labyrynth as quickly as possible, but plenty of people get drawn in.