My area were paying for hardi Plank Smooth $9.13 for 6 1/4 10.60 for 7 1/4 and 12.70 for 8 1/4 12 length. Not cheep stuff.
In Bossier City, LA at Home Depot is shows 144" boards at $7.76 each and her in Alexandria, LA is sells for $5.56 for the 144" boards.
If it's that much cheaper further down south, maybe it's wort ordering from store here and making a trip and haul it to NC. Of course the gas may not make that a feasible option. Just a thought.