I'd be less upset about the amount of board increase than the fact that they decided to give you notice after it was already in effect. As others have said, it's up to the BO to set the price on whatever basis she thinks is reasonable, and up to you to decide whether or not it's worth staying there at that rate.
It sounds like you're not all that thrilled with the barn itself and know of cheaper options anyway, so I'd take it as a cue to give your notice and move elsewhere.
Is your trainer at this barn, or just nearby? If she's at this barn I'd consider tactfully letting her know that you're leaving because of the barn itself and not because of her. If she has clients leaving because the barn isn't being run well or is being overpriced for the services it offers, she will likely pass that kind of feedback on to the BO or move somewhere else herself.