I have boarded at many places over the years, but have never ever had my board vary month to month based on hay or bedding. If a BO ups the board, it is usually a permanent change they do not take lightly,without serious thought. Many only charge more to new boarders, and do not change those of us who have been there. I would never agree to board that is THAT variable. I did have one that actually charged a $25 extra board fee in the winter, but that was reliable and in the original contract. That was as close as I can come to what you do.
So basically your boarders can choose, even tho they PAY for "stall" board, to have their horses out in the rain, or pay more for bedding. *shaking head* Wow.
Bedding is not usually an issue just FYI. Not to mention there are shelters in my pastures if it were raining. However, we keep horses in when its too muddy. The large pastures are less muddy.