Cost per small bale depends a lot on where you are in Kansas. Closer to KC, the higher the price. Further from KC, the cheaper it gets. Our family sells big round bales (1000-1400lb each) for $40 per bale. We haven't done small square in YEARS, but I'm betting they would go for $4-6 per bale now (~60 lb bales).
Equine dental work will by far be your biggest cost. If there was any way to have her mouth looked at BEFORE you buy her, by a vet or dentist, then you would know better what cost you are looking at. Some minis have perfectly fine dental structure and can get by with a exam - sedation - float annually for around $200-$250, maybe more. But if she has a wonky mouth, then her dental costs and care skyrocket.
Farrier will need to come every 6 weeks or so. You could look for a farrier who will teach you how to maintain her feet so that you only have to have a paid farrier visit every 6 months or so. Some are willing to teach clients, some are not. Cost wise, again it will vary by your location.