How much money should I pay?

Those aren't great accommodations, since the field isn't fenced in they aren't safe either. I probably would exchange the work you do for board, I wouldn't pay money for a place like that. Look online and in the paper for other stables nearby. If you are going to invest in owning a horse you should try to find a nicer place for it. If your budget is tight though, board is the least of your worries. Horses are very expensive to keep.