Weith the time and trouble it took to ship in lemons in that time period it sounds fair to me. Off season they had to be preserved in barrels of brine, plus then they had to be shipped to where you want them. Of course if you use citric acid and lemon extract to make it, then the cost would go down.
That's how folks had lemonade on the 4th of July on the Oregon Trail near Independance Rock, lots of accounts of that being part of the celebration.
With me and my crew, costs of transportation is the big one, it ain't cheap to haul a 1/2 ton or more of gear several hundred miles.
Also I know the laws in Kansas and Nebraska and what I have to do, other states you need to find out what hoops we have to jump through and how.
Also we don't haul firewood and I expect plenty of good hardwood, cut and stacked on site when we get there., figure 1/4 to 1/2 a pick-up per day, you have any fire permits worked out before we get there.
Costs per meal depends on what is fixed, 3 meals a day involve about an 18 hour work day, if you want period serving ware, you bring it yourself and wash it, I provide hot water in wreck pans. Coffe, iced tea and/or lemonade are the choices for beverage, tea is brewed fresh, lemonade is made from concentrate from a bottle, PC for the 1870's, lemon extract/citric acid type optional, but more expensive. Hot water and tea provided for those who don't drink coffee.