If the board price is fair, given your area and the facilities that the place have
If the trainer is good, and will actually train the horse
It is a very reasonable price.
Boarding price is hard to judge, because it may vary greatly, only you know your area, the prices that other places have, the facilities that other places have compared to this one (a barn with no arenas will cost way less than another with two outdoor, 1 indoor, round pen, hot showers and paddocks).
300$ for 1 month of training is a good price if the trainer is a good trainer and works the horse well. He should tell you what it is reasonable to expect after said month, what the horse will know that he doesn't now, etc.