Get the mill's nutritionist involved. Tell her what you want, the ingredients you want to use and what your target analysis needs to be as well as cost/ton. How much fat, fiber, protein... do you need to keep your horses in the condition you like? Be flexible to incorporate lesser cost items such as BP, alfalfa pellets, wheat midds, corn gluten (but not 50%!) depending upon upon the market. Add a vitamin/mineral blend if you don't provide it (blocks do not count). Soy oil makes a good binder to make everything stick.
Have them mill the smallest amount they will do for the first run. You might not like the consistancy or it might have too much of X... Some mills will bag but most will only sell bulk feed in 1000-2000# totes. You need to be able to move them and have someplace to store that much product. (1000# is about 4 55# drums full). The trash savings is a huge ++ going bulk.