Back when I never paid more than $2.00 for a pack (tax included) and often paid less, considering frequent discounts, buy one get one free offers, etc., I would say that I would quit if they ever hit $5.00 a pack.
Right now, the cheapest place I can buy them in town is precisely $5.00 a pack, and that's with a $1.00 manufacturer discount. Typically, I pay closer to $6.00, though if I'm stuck far away from my stores of choice, $7.00 or even $8.00 is not unheard of.
So what happened? Simple: before cigarettes cost me $5.00 a pack, they cost me $4.70 or $4.80. When I was paying $2.00, $5.00 was a wickedly steep price increase. By the time it actually jumped to $5.00, though... well, what's another 30 cents? You can lead people anywhere if you're patient enough.