The way BeerSmith is set up, each recipe is self-contained. This way when you make changes in the future to your profiles or ingredients (AA% for hops, for instance) it does not affect the record of recipes already brewed.
Always change the grain pricing in your database if you want it to be a permanent change. Within a specific recipe if it is only a temporary change.
You can always update the prices within a recipe with a singe click. Open the recipe you want to update and in the far right column of options (under the icon of the beer glass) there is an option to '$ update prices'. When you click this option, BeerSmith will update all the pricing data for the ingredients within that recipe.