I bought some Amber Leaf tobacco at Dalaman and it was a few pounds cheaper than on the plane.
I suppose with regards to cigarettes it depends what brand you are after. My friend smokes Marlboro and was paying about £3 a pack in resort if that helps?
Why not have a look at the cost on the plane on the way out then compare with duty free shop on way back to see which is the cheaper?. I don't smoke myself but last year on Jet 2 it was far cheaper for spirits on the plane than in the airport, I paid £18 for a 1 litre bottle of Jack Daniels, well cheap.
And remember the staff at Dalaman will push you to buy the 600 packs of ciggies saying you're allowed to take them back, when you're only allowed to take 200 per person back in to the uk!!!
Not sure who you are flying with but we flew with Thompson this year and the cigs were cheaper on the flight. £60.00 for 600 Lambert and £42.00 for 10 pouches of Amber leaf. Slightly dearer in Dalaman
As a word of caution, I have experienced the brand of cigarettes I wanted running out on the return journey and then you have missed out completely so worth taking this into consideration when making your decision. I have also experienced this on the outbound journey with bottles of Jack Daniels and all comes down to demand as they only have a limited supply. I no longer rely on getting my duty free on the flight.