The simplest solution is to do an UPDATE operation and then INSERT if it fails: I'd create a stored procedure called InsertOrUpdate and do it there:
CREATE PROCEDURE InsertOrUpdate
@ProductName NVARCHAR(MAX),
@Amount INT,
@Price DECIMAL
AS
BEGIN
UPDATE MyTable SET Amount = Amount + @Amount WHERE ProductName = @ProductName
IF @@ROWCOUNT=0
INSERT INTO MyTable (ProductName, Amount, Price) VALUES (@ProductName, @Amount, @Price)
END