We rescued a donkey and a mule (both since had to be put down), and now we have the neighbor's horse in our pasture.
The costs for us have been feed (which fluctuates), regular pedicures, fly spray, worming, and shots (including West Nile in our area).
As the animals got older they started to have more serious health problems and the vet bills got higher. We don't need to board since we had land and considered them our grass-mowing large pets.
Old Merle has started to become real social since he came to us and I am greeted with a whinney every day when I come home from work. Rescuing an animal is a labor of love, but it can get expensive.