Well, it depends. Different places charge different amounts, depends if it's staying a their stable or the trainer will come to you, and how often they will train your horse. I would call local stables and ask around.
I have paid $350 a month for a good trainer come to my farm 4 days a week and work with one horse.
I have also paid $1000 a month for a horse to be at a "show barn" with a nationally known trainer. Shows are extra $$.