Amazon offers Prime members the Hanes Women's Comfortsoft Hipster Panty 6-Pack for $7.24 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $5.48.) That's at least $2 less than you'd spend at another local retail store. They're available in sizes 6 to 9.
Note that although most sizes are currently out of stock, they can still be ordered at this price with an estimated shipping delay of around one week.