while I LOVE Oxbow products and use to feed it exclusively I can't afford it with my gang anymore - we got through a 50# bag of feed here in just under 2 weeks!
It would cost me more to feed our herd Oxbow than it does my human family at this point
If you can afford it, I'd still suggest the smaller bags especially if you're buying direct from Oxbow - they will always send you the freshest stuff they have.
Oxbow does have 50# boxes of hay too btw.
Depending on where you live there's also Kleenmama's stuff that's wonderful too - if you're on the west coast it might work out better for you cost wise
http://www.kmshayloft.com/the people I've known that use her feeds have always been very happy.