Often, I have my own photos and painting materials that I can use. Or, as I travel fairly extensively, often I will be in the neighborhood of a particular place, in the near future. If I can use painting materials that I already have or photos that you have, I generally charge the same amount as I normally would for a painting. If I need to re-visit a particular place, I may need to charge a bit more. Each painting is different.
Watercolors have a mind of their own, of sorts.
I have been doing so, since 1982. If you have an idea, please contact me.