WRONG! I actually thought you might get the price right - my trick was in referring to the meal in question as "dinner." Traditionally, dinner was the big meal of the day and served around noon to 1 pm. At Surratt House, travelers would pay 50 cents for that meal.
Supper was the later and smaller meal of the day, served from 6-7 pm. It would cost 37 1/2 cents at the public dining room in Surratt House (the same cost as breakfast there).
So the answer I was looking for was dinner cost 50 cents. Sister Paige, get out the calculator and see how much we Rebs are winning by now, please. Memo to self: Frisk this Yank the next time he visits the museum.