As accurate as you want it to be.
BeerSmith has pricing available for all ingredients with the exception of water. However, you'll need to update the prices according to your suppliers. What's in BeerSmith represents a generalization of prices from various suppliers.
I've taken the time to update my prices to reflect what my suppliers charge me. I have prices for everything that goes into my recipes, including finings, filer media, spices and minerals. I update the pricing once a year, as needed.