It's really going to depend on what you feed.
For starters, I would price out hay for your area (the price on that varies a LOT from one place to the next) and also consider storage. If you have the storage ability, you can get a better deal (and assure a supply) by buying larger amounts and storing it. If not, you can expect to pay more in the long run and have the worry of not being able to get it when you need it.
We are paying $3-3.50 a bale for really good grass/alf blend bales right now that are about 45-50 lbs per bale. We feed about 1/3 bale a day = $1 - 1.15 a day for hay. During the winter, we feed about 1/2 a bale a day because there isn't grass to graze.
We feed 5 lbs of alfalfa pellets a day - a $13.99 /50 lbs, that works out to 1.40 a day for the pellets.
I feed 1 lb a day rice bran - which works out to .62
Also, during the winter, we may add beet pulp back into the mix if she starts slipping weight - that adds about $1.40 a day.
It really boils down to what you will be feeding. From there, just work it out as above and you'll see what your costs will be. The big thing on hay is whether or not you'll have storage for it.