How can I change NULL to 0 when getting a single value from a SQL function?

Most database servers have a COALESCE function, which will return the first argument that is non-null, so the following should do what you want:

SELECT COALESCE(SUM(Price),0) AS TotalPrice
FROM Inventory
WHERE (DateAdded BETWEEN @StartDate AND @EndDate)

Since there seems to be a lot of discussion about

COALESCE/ISNULL will still return NULL if no rows match, try this query you can copy-and-paste into SQL Server directly as-is:

SELECT coalesce(SUM(column_id),0) AS TotalPrice 
FROM sys.columns
WHERE (object_id BETWEEN -1 AND -2)

Note that the where clause excludes all the rows from sys.columns from consideration, but the ‘sum’ operator still results in a single row being returned that is null, which coalesce fixes to be a single row with a 0.