I pay 6,30E for Dunhill KS on the mainland; 4,65E in Corsica.
B&H are about the same I think, Dunhill International a little bit dearer. French/European cigs are around the 5,70E mark.
Note: unlike the UK, the price of cigs is state-controlled, so you won't get any special offers or anything - you might get a cheap lighter per carton .... Where I get my cigs in Corsica, I buy 10 cartons at a time and usually get a whole load of lighters, notepads, pens, keyrings, fabric bags, & a tee-shirt or sarong or something as well. My savings pay for my trip - and more.
So yes, it's worth buying in France compared to the UK, although I understand that Belgium, Luxembourg and Spain are even cheaper.
PS it's thanks to Napoleon that France mainland can't apply French tax to tobacco products sold in Corsica