A pack of smokes already costs about 2.5 gallons. They will be more expensive than that.
You are probably right. Especially, given the stress of a SHTF scenario, smokes would be an ultra-premium item!
But gas supplies would dwindle rapidly (think Road Warrior) so it would depend on supply and demand, I guess.
In general, I think post-SHTF, driving anywhere might be a very dangerous proposition.