A lot of that depends on where you live. If you can let the horse eat grass as much as possible then you won't need hay for part of the year. If its on hay all year that can get expensive in some areas and in some years. I don't know how much a mini eats but I know one of my horses would eat 1 bale in about 2 days and bales can run anywhere from $2.00 to $6-8
Farrier cost me $30 per horse every 6 weeks I'm sure a mini would cost the same since its the time and trip, not so much the size of the hoof.
Vet bills I can't tell because mine are always mixed in with something else and its for 4 horses.
A bag of pellet feed is around $19-20 and mine get 1 lb per day so figure less than that for yours a 50lb bag will last of course 50 days for one of my horses so it would last much longer for you.
A 10x10 stall is all that many horses get, but its not great for their well being or mental health. The more you can give it the better. Plus there would be the added benefit of where the horse is that's less you have to mow in the summer