try this if table has record but price is null
Select (ISNULL(Tbl_StartupBanner.Price,0) + ISNULL(Tbl_SignInBanner.Price,0) + ISNULL(Tbl_SignOutBanner.Price,0)) as Total from Tbl_StartupBanner,Tbl_SignInBanner,Tbl_SignOutBanner
if table has no records try this
SELECT( (Select ISNULL(SUM(Tbl_SignOutBanner.Price),0) FROM Tbl_SignOutBanner) +
(Select ISNULL(SUM(Tbl_SignInBanner.Price),0) FROM Tbl_SignInBanner)+
(Select ISNULL(SUM(Tbl_StartupBanner.Price),0) FROM Tbl_StartupBanner)
) as total