I honestly don't think there is any need to shoot any shot size larger than 6's in the denser than lead shells. The 6's will outpattern the 4's and 5's by a wide margin and maintain lethal killing power well past the point that 4's or 5's will hold an acceptable pattern.

As for price, once your gun is patterened with them you will only be shooting turkeys with them. How many turkeys you shoot a year? Even if I killed my limit every year here in Alabama a box of ten would still last two years. With 40 dollar Winchester Elite extended range shells that's a yearly ammo cost of 20 bucks. And that's not much.

I also second the Indian Creek Choke recommendation.