Depends on location.
A pack contains twenty cigarettes. A carton is ten packs. Smokes purchased by the carton are cheaper per cigarette than packs purchased individually.
In Colorado (which has "average" prices nationwide), a pack of premium-brand cigarettes go for around $3.75/pack. Cartons can be around $30 --sometimes less if stores have them on sale. (Premium brands are the non-generic, "popular" brands like Marlboros, Camels, Winstons, Salems, Benson & Hedges, etc...) Generic or "discount" brands like Basics and Dorals can go for under $3 per pack. Many cigarette manufactures run promotions like "buy two packs get one free" to try and compete.
I almost always buy whatever brand is on sale or part of a promotion.
New York City is currently making headlines as having the highest price for smokes-- $7+ per pack. In Ohio, it is a felony to posess more than $60 worth of smokes that do not contain an Ohio tax stamp (cartons run for $40 in Ohio but people have been fleeing to Kentucky where they are just over $20).
Hope this helps.