Arielle,
I know your post about designing a silo is from back in December but I wanted to suggest, if you where still looking, search out used cattle/chicken feed bins, they sit up on 4 legs, have a holding capacity of about 500 lbs up 10-20 tons, obviously you wouldn't need to fill one all the way, we run 13 cattle in a 10 ton bin but only get about 2 tons per delivery. They vary greatly in price but I see them frequently below $500. Put some galvanized paint on the tops to make sure its sealed up and pour 4 piers for the legs (or just a platform) and call it good. I am only about 30 minutes from our mill and work commute takes my wife right past it so we can just pick up as needed but in your situation, it may be worth while investigating! One thought on bulking buying any feed though, like bread, it does go stale regardless of how you keep it, so you don't want to buy more than about a months worth.