About Count from different tables

You need to use CTE for this

--TableA
CREATE TABLE [dbo].[TableA](
	[Id] [int] IDENTITY(1,1) NOT NULL,
	[Price] [money] NOT NULL,
 CONSTRAINT [PK_TableA] PRIMARY KEY CLUSTERED 
(
	[Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
--TableB
CREATE TABLE [dbo].[TableB](
	[Id] [int] IDENTITY(1,1) NOT NULL,
	[Price] [int] NOT NULL,
 CONSTRAINT [PK_TableB] PRIMARY KEY CLUSTERED 
(
	[Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
--INSERT STATEMENT
INSERT INTO [TableB]([Price])
     VALUES(10)
INSERT INTO [TableB]([Price])
     VALUES(20)
INSERT INTO [TableA]([Price])
     VALUES(30)
INSERT INTO [TableA]([Price])
     VALUES(40)
--CTE
;WITH CTE1 AS
(
     SELECT SUM(tableA.Price) AS Price FROM TableA
),
CTE2 AS
(
    SELECT SUM(tableB.Price) AS Price FROM TableB
)
SELECT CTE1.Price +  CTE2.Price
FROM CTE1, CTE2

Thank You.