Don't worry about ammo price too much, you have your 5.56 for blasting. Build up something a bit off the wall. That stuff is always fun. My vote is for a 25-45 Sharps, or 6.5 Grendel.
My vote would be for the 25-45 Sharps. Those 100gr Swine Smasher rounds dropped those pigs dead in their tracks.
They have several other loads as well for other uses.
http://store.srcarms.com/category-s/1830.htm
http://srcarms.com/wordpress/25-45-sharps/ammunition/