Musician's Friend offers the D'Addario EXP16 Coated Phosphor Bronze Light Acoustic Guitar Strings for $11.46. Better yet, add two sets to your cart and coupon code "EXPBOGO" makes the second set free. With free shipping, it's the lowest total price we could find for two sets by $9.
On a similar note, you can get two sets of D'Addario EXP17 Coated Phosphor Bronze Medium Acoustic Guitar Strings for $12.29 after the above coupon, also with free shipping. That's another price low, this time by $8.