I don't charge anything extra for board for a mare and foal combo until the foal is weaned...I then charge what I would charge any other horse, as its requires the same amount of time and handling as any other boarded horse.
From the time the mare foals till the time the foal is weaned they are treated as one...as they share a stall, they are turned out together etc.
As for mare care...I don't charge anything extra for the most part....as any costs required such as vaccinations, ultrasounds are charged from the vet straight to the boarder. Any special feed that may be required is bought, but charged appropriately to the boarder.
Now this being said, if a mare did require additional care above and beyond what I would deem normal care, then perhaps I would charge an additional fee to off set any extra time and effort.
Horses are like chips...you can never just have one